Update bridging docs

[HarHarLinks]

https://matrix.org/docs/communities/bridging/ uses a pretty arbitrary example instead of a comprehensive overview about bridging.

I propose we should

• explain the concept of bridging, different types of bridging with their respective requirements, etc. backporting https://matrix.org/docs/older/types-of-bridging/ while verifying its contents would be a good starting ground.
• delineate bridges and integrations (e.g. hookshot calls itself a bridge, whereas we call it an integration)
• list the public bridges offered by the Foundation
• list other well-known public bridges, incl t2bot.io, arianet, ..?
• perhaps keep the existing contents as an example

we can collect some more cornerstones of such an article here before writing a draft.