Organize Meetings page into subpages

[sapols]

This PR cleans up our Meetings page by grouping things into subpages.

Now, everything that used to be below the calendar has been moved into one of 3 subpages:

• Community Meetings
• Conference Sessions
• Summer Schools

These subpages are now listed in-between our telecon info and the calendar.

Implementation notes:

• The subpage links are H3s like the other two headers on the page.
• I waffled between single- or double-spacing the subpage links. It's single-spaced right now. Double spacing visually separates the links more, but it looked like a whole lot of whitespace. Do you have any preference either way?
• The refactor involved shuffling around file locations:
  • Now, rather than dumping every .md file into the meetings/ folder, they go in one of 3 subfolders (community_meetings/, conference_sessions/, summer_schools/).
  • I had to fix a bunch of relative links after I moved those files.

[sapols]

@jibarnum is this better? With Events?

I'd specifically wrestled with this idea when I was making this PR, but initially decided against it because any header I could think of was either redundant or inaccurate. (1) It's already the "Meetings" page so no point in having a second header that says "meetings", (2) Telecons are also "PyHC relevant meetings" so we can't use that, (3) "Events" is the best I can come up with, but "Community Meetings" are really more "meeting" than "event." "Other Meetings" might be more accurate but that sounds dumb.

[jibarnum]

@jibarnum is this better? With Events?

I'd specifically wrestled with this idea when I was making this PR, but initially decided against it because any header I could think of was either redundant or inaccurate. (1) It's already the "Meetings" page so no point in having a second header that says "meetings", (2) Telecons are also "PyHC relevant meetings" so we can't use that, (3) "Events" is the best I can come up with, but "Community Meetings" are really more "meeting" than "event."

Totally, I can see how adding Meetings again in there is redundant. I just wanted something there to break up the flow. Even though meetings are more meetings than they are events, I still like the title "Events".