Adding participate section and other minor edits

Adding participate section and other minor edits

1. Removing some of the repetitive bullet points from the main page.
2. Adding detail participate section to the main page
3. Fixing some formatting.
4. Removing references to the wiki page for AGL distrou and google calendar

Bug-AGL: [SPEC-4470]
Signed-off-by: Vinod Ahuja <vahuja@unomaha.edu>
Change-Id: I82d581ca05d6e0ee824465631c4e404caf2e4f86
Signed-off-by: Vinod Ahuja <vahuja@unomaha.edu>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/documentation/+/27793
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Tested-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>