Doc: Update for 2.1.0 release

* Add new proposal to "changes"
* Better organize home page
* Update download text for BAMMtools
* Add note on how to test BAMM can run
* Add Homebrew installation instructions
* Installation doc is for post-download
* Update minimum version to 10.7.5
* Update download links to v2.1.0
* Update links to previous versions
* Add Homebrew installation text
* Add MC3 to contents and re-order contents