apidocs: add logo, backlink, alabaster theme

[kba]

In preparation of rebuilding the API Docs on the website as a standalone website:

• Uses the alabaster theme
• disables the service links (so the search field isn't hidden all the way at the bottom)
• replaces the title with the OCR-D logo which links back to the OCR-D homepage (the same behavior as with the GT guidelines once Guidelines standalone ocrd-website#229 is merged)
• Now also generates a nice documentation of the CLIs, thanks to sphinx_click (ht @bertsky)