Add rtd config file and refactor test / docs dependencies

[yakky]

Description

Move RTD configuration to config file

Simplify test / docs dependencies

References

Fix #624

Checklist

• I have read the contribution guide
• Code lint checked via inv lint
• changes file included (see docs)

[codecov]

Codecov Report

Merging #625 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #625   +/-   ##
========================================
  Coverage    94.14%   94.14%           
========================================
  Files           22       22           
  Lines         1041     1041           
  Branches       137      137           
========================================
  Hits           980      980           
  Misses          23       23           
  Partials        38       38           

Flag	Coverage Δ
#unittests	94.14% <ø> (ø)

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[github-actions]

Coverage remained the same at 96.582% when pulling f019f6a on feature/issue-624-rtd-config into 4be02ca on develop.