Install page: Show toc and add older versions link

Modified the install page so that it now shows:
- The table of contents at the beginning of the page. Had to modify the
markdown a bit to make this appear (it only shows it if there's no first level
heading in the md file).
- Added a new section to tell users where they can find the installers/binaries
for older versions of Pidgin/Finch.

Testing Done:
Ran `dev-server.sh` and verified content looked as intended

Reviewed at https://reviews.imfreedom.org/r/834/