doc: change the sphinx theme from default to sphinx_rtd_theme

Closes: #926336
3 jobs for <span class="ref-name">feat-change-sphinx-theme-to-sphinx_rtd_theme</span> in 7 minutes and 43 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Test
passed #152788
flake8

00:04:18

passed #152790
functional-tests

00:07:42

passed #152789
unit-tests

00:06:26

86.0%