docs: Use read the docs theme if available

Merged Bitbucket Importer requested to merge bitbucket/merged-pr-230 into branch/default

Created originally on Bitbucket by techtonik (anatoly techtonik)

Was already merged in Bitbucket before import, marked as merged by the import user

This makes docs built locally look the same as on RTD

https://github.com/snide/sphinx_rtd_theme#using-this-theme-locally-then-building-on-read-the-docs

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