docs/requirements.txt: add sphinx-rtd-theme

[olichtne]

Description

adding a theme dependency to hopefully fix the readthedocs build.

Tests

(Please provide a list of tests that prove that the pull request doesn't break the stable state of the master branch. This should include test runs with valid results for all of critical workflows.)

Reviews

(Please add a list of reviewers that should check the validity and sanity of this merge request before it's accepted. Use the @username syntax. If you don't know who to mention just link @all.)

Closes: #

[olichtne]

no review needed, the readthedocs ci test passing is enough.