Improve and modernize docs. (#2882)

* Add proper home landing page.

* Improve landing page considerably.

* Add navbar library name.

* Fix css rules.

* Update getting started images.

* Change favicon to the navbar's logo.

* Tiny change to docs index.

* Add title to home landing page.

* Tweak build overrides faq page.

* Fix FAQ article titles and headers.

* Fix FAQ article titles and headers.

* Replace channel 9 with the official .net course.

docs/faq/toc.yml

@@ -22,7 +22,7 @@
   items: 
     - name: Text Command basics
       topicUid: FAQ.TextCommands.General
-- name: Legacy or Upgrade
+- name: Legacy Questions
   topicUid: FAQ.Legacy
 - name: Build Overrides
   topicUid: FAQ.BuildOverrides.WhatAreThey