Revise sidebarwidgets.rst to add details on Largo widgets, and fix errors

[jackbrighton]

The sidebarwidgets page on http://largo.readthedocs.org/users/sidebarswidgets.html was woefully incomplete and out of date, and revising it has been a high priority prior to the coming release of Largo 0.5.5. (see: #847)

Legacy language and formatting was cleaned up considerably and normalized. Sections were added for Largo widgets that weren't in the docs, and other sections were revised for accuracy, spelling, language, etc. Formatting was applied for better usability. Basically this is a complete revamp of the page.

It was suggested to create pages for each widget, so as to provide more details, screenshots, etc., and that can certainly be done. Since it was a high priority to fill in missing information and to correct errors, I chose to first make these changes to the existing page.

A readable version of the revised page can be found here: https://github.com/inn/docs/blob/master/how-to-work-with-us/pull-requests.md

[aschweigert]

A couple small issues (will make this a separate ticket, but noting here for reference):

• if we add the link roundups widget we should also add the saved links widget (added by the same plugin
• this should actually probably be a separate section including widgets added by commonly used plugins (link roundups, dfp, analytic bridge, etc.)
• we need to add a section re: the default WP widgets that we de-register (and maybe explain why), the list can be found in inc/widgets.php
• the taxonomy list widget will need updated docs when Revisit the Taxonomy List widget #1171 is merged