Remove docs/sphinx and related references (#8480)

https://github.com/matrix-org/synapse/tree/develop/docs/sphinx doesn't seem to really be utilised or changed recently since the initial commit. I like the idea of exportable documentation of the codebase, but at the moment after running through the build instructions the generated website wasn't very useful...

docs/sphinx/synapse.api.handlers.rst

-synapse.api.handlers package
-============================
-
-Submodules
-----------
-
-.. toctree::
-
-   synapse.api.handlers.events
-   synapse.api.handlers.factory
-   synapse.api.handlers.federation
-   synapse.api.handlers.register
-   synapse.api.handlers.room
-
-Module contents
----------------
-
-.. automodule:: synapse.api.handlers
-    :members:
-    :undoc-members:
-    :show-inheritance:

docs/sphinx/synapse.api.notifier.rst

-synapse.api.notifier module
-===========================
-
-.. automodule:: synapse.api.notifier
-    :members:
-    :undoc-members:
-    :show-inheritance:

docs/sphinx/synapse.api.register_events.rst

-synapse.api.register_events module
-==================================
-
-.. automodule:: synapse.api.register_events
-    :members:
-    :undoc-members:
-    :show-inheritance:

docs/sphinx/synapse.api.room_events.rst

-synapse.api.room_events module
-==============================
-
-.. automodule:: synapse.api.room_events
-    :members:
-    :undoc-members:
-    :show-inheritance:

docs/sphinx/synapse.api.rst

-synapse.api package
-===================
-
-Subpackages
------------
-
-.. toctree::
-
-    synapse.api.events
-    synapse.api.handlers
-    synapse.api.streams
-
-Submodules
-----------
-
-.. toctree::
-
-   synapse.api.auth
-   synapse.api.constants
-   synapse.api.errors
-   synapse.api.notifier
-   synapse.api.storage
-
-Module contents
----------------
-
-.. automodule:: synapse.api
-    :members:
-    :undoc-members:
-    :show-inheritance:

docs/sphinx/synapse.api.server.rst

-synapse.api.server module
-=========================
-
-.. automodule:: synapse.api.server
-    :members:
-    :undoc-members:
-    :show-inheritance:

docs/sphinx/synapse.api.storage.rst

-synapse.api.storage module
-==========================
-
-.. automodule:: synapse.api.storage
-    :members:
-    :undoc-members:
-    :show-inheritance:

docs/sphinx/synapse.api.stream.rst

-synapse.api.stream module
-=========================
-
-.. automodule:: synapse.api.stream
-    :members:
-    :undoc-members:
-    :show-inheritance:

docs/sphinx/synapse.api.streams.event.rst

-synapse.api.streams.event module
-================================
-
-.. automodule:: synapse.api.streams.event
-    :members:
-    :undoc-members:
-    :show-inheritance:

docs/sphinx/synapse.api.streams.rst

-synapse.api.streams package
-===========================
-
-Submodules
-----------
-
-.. toctree::
-
-   synapse.api.streams.event
-
-Module contents
----------------
-
-.. automodule:: synapse.api.streams
-    :members:
-    :undoc-members:
-    :show-inheritance:

docs/sphinx/synapse.app.homeserver.rst

-synapse.app.homeserver module
-=============================
-
-.. automodule:: synapse.app.homeserver
-    :members:
-    :undoc-members:
-    :show-inheritance:

docs/sphinx/synapse.app.rst

-synapse.app package
-===================
-
-Submodules
-----------
-
-.. toctree::
-
-   synapse.app.homeserver
-
-Module contents
----------------
-
-.. automodule:: synapse.app
-    :members:
-    :undoc-members:
-    :show-inheritance:

docs/sphinx/synapse.db.rst

-synapse.db package
-==================
-
-Module contents
----------------
-
-.. automodule:: synapse.db
-    :members:
-    :undoc-members:
-    :show-inheritance:

docs/sphinx/synapse.federation.handler.rst

-synapse.federation.handler module
-=================================
-
-.. automodule:: synapse.federation.handler
-    :members:
-    :undoc-members:
-    :show-inheritance:

docs/sphinx/synapse.federation.messaging.rst

-synapse.federation.messaging module
-===================================
-
-.. automodule:: synapse.federation.messaging
-    :members:
-    :undoc-members:
-    :show-inheritance:

docs/sphinx/synapse.federation.pdu_codec.rst

-synapse.federation.pdu_codec module
-===================================
-
-.. automodule:: synapse.federation.pdu_codec
-    :members:
-    :undoc-members:
-    :show-inheritance:

docs/sphinx/synapse.federation.persistence.rst

-synapse.federation.persistence module
-=====================================
-
-.. automodule:: synapse.federation.persistence
-    :members:
-    :undoc-members:
-    :show-inheritance:

docs/sphinx/synapse.federation.replication.rst

-synapse.federation.replication module
-=====================================
-
-.. automodule:: synapse.federation.replication
-    :members:
-    :undoc-members:
-    :show-inheritance:

docs/sphinx/synapse.federation.rst

-synapse.federation package
-==========================
-
-Submodules
-----------
-
-.. toctree::
-
-   synapse.federation.handler
-   synapse.federation.pdu_codec
-   synapse.federation.persistence
-   synapse.federation.replication
-   synapse.federation.transport
-   synapse.federation.units
-
-Module contents
----------------
-
-.. automodule:: synapse.federation
-    :members:
-    :undoc-members:
-    :show-inheritance:

docs/sphinx/synapse.federation.transport.rst

-synapse.federation.transport module
-===================================
-
-.. automodule:: synapse.federation.transport
-    :members:
-    :undoc-members:
-    :show-inheritance: