Make extensions support parallel reading

[amotl]

Problem

At #648 (comment), we discovered that a few of the extensions the theme is using do not support parallel reading (yet).

Details

• sphinx-subfigure has been removed with the PR referenced above, because we don't need it.

• sphinx-csv-filter is apparently marked safe for parallel reading already, but has a dedicated ticket now to process any problems, see Support parallel reading for Sphinx sphinx-csv-filter#51.

• oembedpy is also marked safe for parallel reading, but apparently needs improvements, so we've forwarded the report, see Sphinx: Problem with parallel reading attakei/oEmbedPy#12.

[amotl]

Updates

• CrateDB/blackbox needs to update to sphinx-csv-filter>=0.4.1 to have parallel reads.
• Because we found no use of the oembedpy extension, we just removed it per Remove oembedpy extension #650.

[amotl]

Build time: 59.53 secs -> 8.60 secs

@mfussenegger reported about significant improvements at #650 (comment), thanks.
Let's run a release now, and also add -j auto to crate-docs?

[amotl]

crate-docs now also provides parallel builds. Please use the most recent version 2.1.5.

• Enable parallel builds crate-docs#119
• Enabled parallel builds using --jobs auto also for link checker runs crate-docs#120