Add an RSS feed

[ashryanbeats]

https://github.com/withastro/astro/tree/main/packages/astro-rss

It should pick up any new content, including:

1. Recently published community content
2. Events
3. Community member projects

Note that this won't be as straightforward as it may appear because each of the above items is quite different in how they behave, so it could be we should split this issue into:

1. A primary RSS scaffolding issue, and
2. Separate issues for each content type