Regenerate the gitwash docs.

[aashil]

Regenerate the gitwash docs and add README.txt documenting how to use the script to generate the docs. Fixes #8123

[NelleV]

Can you add a link to the gitwash script?

[aashil]

I restructured the sentence a bit and added the link to gitwash_dumper.py script. Have a look.

[NelleV]

Apart from my comment, it looks good to me.

[QuLogic]

Documentation isn't building:

Warning, treated as error:

/home/travis/build/matplotlib/matplotlib/doc/devel/gitwash/setting_up_for_development.rst:: WARNING: document isn't included in any toctree

[matthew-brett]

It looks like the setting_up_for_development page is a version of gitwash's git_development page, and can be deleted.

[QuLogic]

Still more doc errors:

Warning, treated as error:

/home/travis/build/matplotlib/matplotlib/doc/devel/gitwash/dot2_dot3.rst:: WARNING: document isn't included in any toctree

[aashil]

Any idea why appveyor and codecov fail?

[phobson]

@aashil they're both kind of finiky. I looks like appveyor had some transient network issues. I restarted its build.

[dstansby]

The fancy log bit is over-running the text box quite a bit, does anyone know if there's a way to fix this?

[QuLogic]

It's a literal block, so I think the only choice is to pick a shorter set of log lines.

[dstansby]

Not sure if this wants a backport - if anyone else thinks it does feel free to do the backport.

[tacaswell]

Backported to v2.0.0-doc as 1d87fba