mailing_lists.md: add French IRC channel
mailing_lists.md: add intro with info on other lists and forums
community/index.md: change link to mailing lists

Was to lists.ocaml.org, which only covers small set of all lists. Now
linking to main mailing lists page.
community/index.md: change Events to Meetings

Section links to Meetings page, and we refer to it as “meetings”
everywhere else.