Building settings list from docs

[eliasdorneles]

Hey, folks!

So, this PR intends to fix #518, changing the scrapydocs extension to do the following:

• collect all setting definitions in the documentation in the env variable
• adds a directive settingslist which will render a list of settings defined anywhere in the documentation except in the current document (I added this restriction to make it less verbose, the complete list is too big).

Does this look good?

[pablohoffman]

It's looking good, just two suggestions:

1. can we show the list sorted alphabetically?
2. can we put them in a proper HTML <ul> list?. Right now each one is on a separate paragraph <p> and that makes the rendering a bit awkward.

[eliasdorneles]

@pablohoffman done!

[pablohoffman]

good job @eliasdorneles !