Use readthedocs theme for docs
Add updated font installation docs
Remove old docs and update documentation structure

Most of the files are empty and need to be copied or rewritten from the
old docs. Font patching docs have been removed entirely and will be
moved to the powerline-fontpatcher repo.

Ref #769
Copy documentation from old variant

Note: fontpatcher documentation was removed.

Closes #632
Closes #769
Update terminal support matrix

Fixes #763
Closes #660
Remove timeout tip

Closes #808
Closes #266
Add note about ``tmux -2``

Closes #847
Increase depth to 3

Reason: this way I have anything I want in one click. Less precise toctree is in
any case present in the side bar.
Add note about `https://` protocol

Closes #881
Added Source Code Pro documentation.rst

Added a short blurb on Source Code Pro and how to make the glyphs show up.

Closes #460
Fix styling of rxvt-unicode tips
Fix segment_data key description in configuration reference
Add font installation instructions
Add code for Vundle installations and related warning

Ref #414
Closes #378
Add note about automatic vimrc resourcing

Ref #297
Ref #296
Closes #213
Add note about minibufexpl

Closes #230
Merge pull request #945 from ZyX-I/update-docs

Update documentation