[feat] Minimize the list of doc requirements

[teojgo]

Fixes #1836

[vkarak]

Is this indeed the minimal? I.e., if I remove one those, the build will fail? In the Spack package we have only those and it seems to work:

https://github.com/spack/spack/blob/3b1b51e90de380fa9524121f95df989b00eeb6a8/var/spack/repos/builtin/packages/reframe/package.py#L66-L68

[teojgo]

Is this indeed the minimal? I.e., if I remove one those, the build will fail? In the Spack package we have only those and it seems to work:

https://github.com/spack/spack/blob/3b1b51e90de380fa9524121f95df989b00eeb6a8/var/spack/repos/builtin/packages/reframe/package.py#L66-L68

Well performing pip install sphinx brings almost all the above with it.

[codecov-io]

Codecov Report

Merging #1847 (bd316bc) into master (a3d0b0c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1847   +/-   ##
=======================================
  Coverage   87.61%   87.61%           
=======================================
  Files          49       49           
  Lines        8136     8136           
=======================================
  Hits         7128     7128           
  Misses       1008     1008           

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