Fixing broken links in descriptions for Get Involved pages

Adds a !SITE_BASEURL! constant for internal links within page
descriptions
Fixes #2 hgroup removed from HTML5 specification
Tidying up some more pages to continue to fix #2

Also cleaning up primary CSS file
Add Version Check
Updating team members
Include release checksums

These are static files so Github Pages can serve them. No modifications to MyBB itself are needed.
Remove meta keywords
Adding initial set of support guidelines
Replace mybboard.net with mybb.com
Replace "wiki" with "MyBB Documentation"

Also replace wiki.mybb.com with docs.mybb.com. I've left some links as we don't have articles for them yet.
Add correct links to Merge Support
Update minimum requirements
Fix `baseurl`

Blargh.
Begins addressing #7 Issue appropriate redirects

Fixes existing redirects and adds some of those that were missing; more
may still be missing
Adds a 404 page with Google site search
WIP Replace screenshots

This is my first round replacing the screenshots which can be found around the site. I've left some as I need to find the best way to scale them down without losing too much quality.

Although I assume Justin was going to do this, OSX likes to render fonts differently than Windows/Linux and overall MyBB looks better when viewed from these systems. I don't believe this is something we can control from the MyBB side, though.
Update MyBB Mods URL/branding
Update Merge System version
Add bbPress to merge modules list
Update MyBB Mods URL