Fixes for various issues in "building" doc:
- removed information about mercurial and replaced with git as appropriate (as mercurial repos are no longer active)
- fixed broken section references
- fixed broken links, updated to web archive, where original site no longer exists ( with exception of "Windows FreeType DLL build instructions" link, where archived page is not available :( )
- updated md tables to use "|", so they would correctly display in GH
- avoided use of block quotes (using ">") in md, as it produces ugly result in GH
- regenerated html (visual changes in generated html are quite minor)