[docs] many improvements to the documentation / cleanup (#4817)

* fixed RSTs so images will show up on github
* fresh screenshots on main page
* removing irrelevant portions
* moved a set of sections under 'Misc'
* rebuilt the Gallery with all screenshots

docs/videos.rst

@@ -1,54 +1,5 @@
 Videos
 ======
 
-Here is a collection of short videos showing different aspect
-of Superset.
-
-Quick Intro
-'''''''''''
-This video demonstrates how Superset works at a high level, it shows how
-to navigate through datasets and dashboards that are already available.
-
-.. youtube:: https://www.youtube.com/watch?v=3Txm_nj_R7M
-
-Dashboard Creation
-''''''''''''''''''
-This video walk you through the creation of a simple dashboard as a
-collection of data slices.
-
-- Coming soon!
-
-Dashboard Filtering
-'''''''''''''''''''
-This video shows how to create dynamic filters on dashboards, how to
-immunize certain widgets from being affected by filters.
-
-- Coming soon!
-
-Customize CSS and dashboard themes
-''''''''''''''''''''''''''''''''''
-A quick walkthrough on how to apply existing CSS templates, alter them and
-create new ones.
-
-- Coming soon!
-
-Slice Annotations
-'''''''''''''''''
-A short video on how to annotate your charts, the markdown language and
-to toggle them on dashboards.
-
-- Coming soon!
-
-Adding a Table
-''''''''''''''
-This videos shows you how to expose a new table in Superset, and how to
-define the semantics on how this can be accessed by others in the ``Explore``
-and ``Dashboard`` views.
-
-- Coming soon!
-
-Define SQL Expressions
-''''''''''''''''''''''
-A walkthrough on how to create your own derived dimensions and metrics.
-
-- Coming soon!
+.. note ::
+    This section of the documentation has yet to be filled in.