This only fixes Markdown syntax, and adds a few headers to bring
some structure into the long list of variables that influence the
build. It does not add or remove variables.
* Fix more broken links
* Move FAQ last
* Fold all install-y content into `install.md`
* Fix header levels
* Fold `download.md` contents into `install.md`
* Fix cross-references
* Ordered navigation
* Add logo to docs
* Make doc pages more consistent