Builds docs in parallel

[rmartin16]

Changes

• With the release of sphinx-csv-filter v0.4.1, Toga docs can be built in parallel.

Notes

• Using the -r\--recreate flag for tox may be necessary to update sphinx-csv-filter.
• This should drastically speed up builds on many core machines.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[rmartin16]

So, personally, I would classify filesystem as a word (although as I type this, apparently even Chrome disagrees with this as does the sphinx-spelling dictionary). Nonetheless, I see #1881 already exists to update the docstring that's failing the RTD build here. If that's merged, this build should succeed.

[freakboy3742]

I've just merged #1881; so this should now pass docs CI.