Remove meetings agendas/minutes (moved to separate repo). (#4388)

This PR removes all minutes and agendas in `meetings/`. These were
previously hosted in this repository, but we found that it makes things
somewhat more complex with respect to CI configuration and merge
permissions to have both small, CI-less changes to the text in
`meetings/` as well as changes to everything else in one repository.

The minutes and agendas have been split out into the repository at
https://github.com/bytecodealliance/meetings/, with all history
preserved. Future agenda additions and minutes contributions should go
there as PRs.

Finally, this PR adds a small note to our "Contributing" doc to note the
existence of the meetings and invite folks to ask to join if interested.

.github/workflows/main.yml

@@ -3,12 +3,8 @@ on:
   push:
     branches: [main]
     tags-ignore: [dev]
-    paths-ignore:
-      - 'meetings/**'
   pull_request:
     branches: ['main', 'release-*']
-    paths-ignore:
-      - 'meetings/**'
 defaults:
   run:
     shell: bash