Add content to the Synapse documentation intro page (#10990)

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

changelog.d/10990.doc

+Add additional content to the Welcome and Overview page of the documentation.

docs/development/contributing_guide.md

@@ -63,7 +63,7 @@ TBD
 
 # 5. Get in touch.
 
-Join our developer community on Matrix: #synapse-dev:matrix.org !
+Join our developer community on Matrix: [#synapse-dev:matrix.org](https://matrix.to/#/#synapse-dev:matrix.org)!
 
 
 # 6. Pick an issue.

docs/welcome_and_overview.md

@@ -3,3 +3,77 @@
 Welcome to the documentation repository for Synapse, a 
 [Matrix](https://matrix.org) homeserver implementation developed by the matrix.org core 
 team.
+
+## Installing and using Synapse
+
+This documentation covers topics for **installation**, **configuration** and
+**maintainence** of your Synapse process:
+
+* Learn how to [install](setup/installation.md) and
+  [configure](usage/configuration/index.html) your own instance, perhaps with [Single
+  Sign-On](usage/configuration/user_authentication/index.html).
+
+* See how to [upgrade](upgrade.md) between Synapse versions.
+
+* Administer your instance using the [Admin
+  API](usage/administration/admin_api/index.html), installing [pluggable
+  modules](modules/index.html), or by accessing the [manhole](manhole.md).
+
+* Learn how to [read log lines](usage/administration/request_log.md), configure
+  [logging](usage/configuration/logging_sample_config.md) or set up [structured
+  logging](structured_logging.md).
+
+* Scale Synapse through additional [worker processes](workers.md).
+
+* Set up [monitoring and metrics](metrics-howto.md) to keep an eye on your
+  Synapse instance's performance.
+
+## Developing on Synapse
+
+Contributions are welcome! Synapse is primarily written in
+[Python](https://python.org). As a developer, you may be interested in the
+following documentation:
+
+* Read the [Contributing Guide](development/contributing_guide.md). It is meant
+  to walk new contributors through the process of developing and submitting a
+  change to the Synapse codebase (which is [hosted on
+  GitHub](https://github.com/matrix-org/synapse)).
+
+* Set up your [development
+  environment](development/contributing_guide.md#2-what-do-i-need), then learn
+  how to [lint](development/contributing_guide.md#run-the-linters) and
+  [test](development/contributing_guide.md#8-test-test-test) your code.
+
+* Look at [the issue tracker](https://github.com/matrix-org/synapse/issues) for
+  bugs to fix or features to add. If you're new, it may be best to start with
+  those labeled [good first
+  issue](https://github.com/matrix-org/synapse/issues?q=is%3Aissue+is%3Aopen+label%3A%22good+first+issue%22).
+
+* Understand [how Synapse is
+  built](development/internal_documentation/index.html), how to [migrate
+  database schemas](development/database_schema.md), learn about
+  [federation](federate.md) and how to [set up a local
+  federation](federate.md#running-a-demo-federation-of-synapses) for development.
+
+* We like to keep our `git` history clean. [Learn](development/git.md) how to
+  do so!
+
+* And finally, contribute to this documentation! The source for which is
+  [located here](https://github.com/matrix-org/synapse/tree/develop/docs).
+
+## Donating to Synapse development
+
+Want to help keep Synapse going but don't know how to code? Synapse is a
+[Matrix.org Foundation](https://matrix.org) project. Consider becoming a
+supportor on [Liberapay](https://liberapay.com/matrixdotorg),
+[Patreon](https://patreon.com/matrixdotorg) or through
+[PayPal](https://paypal.me/matrixdotorg) via a one-time donation.
+
+If you are an organisation or enterprise and would like to sponsor development,
+reach out to us over email at: support (at) matrix.org
+
+## Reporting a security vulnerability
+
+If you've found a security issue in Synapse or any other Matrix.org Foundation
+project, please report it to us in accordance with our [Security Disclosure
+Policy](https://www.matrix.org/security-disclosure-policy/). Thank you!