General documentation improvements
General documentation improvements

* Made documentation levels consistent
* Added a (manually generated) TOC (will probably get out of date, but it was helpful)
* Rearanged quick links and added new links
* Moved project overview to top for use for those just browsing

cr1901/HDMI2USB-litex-firmware,mithro/HDMI2USB-litex-firmware,cr1901/HDMI2USB-litex-firmware,mithro/HDMI2USB-litex-firmware,mithro/HDMI2USB-litex-firmware,cr1901/HDMI2USB-litex-firmware,mithro/HDMI2USB-litex-firmware,cr1901/HDMI2USB-litex-firmware