[doc][s]: upgrade to new version of sphinx theme plus some minor reworking.

* Switch master_doc back to front page (avoids bad UX and weird bread-crumb behaviour) - see #1325 comments
* This then meant toc not generating as front page has no toc directive. So merged contents.rst back into front page (good idea UX wise any way ... (?))
* Tweak titles for documentation (partly to make breadcrumb nicer)
* Fix up logo to point to one in assets (nicer) - and remove local doc copy of logo