Revive rtd docs

[jgliss]

Closes #46

Things done:

• [x] Enable RTD builds on PRs
• [x] Fix setup and required env for building sphinx docs on RTD
• [x] Add cron job to run CI workflow on Mondays at 12:00 (will only take effect on next release)

[codecov-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (490ac6b) 35.03% compared to head (883810c) 35.03%.

:exclamation: Your organization is not using the GitHub App Integration. As a result you may experience degraded service beginning May 15th. Please install the Github App Integration for your organization. Read more.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## v145dev #47 +/- ## ======================================== Coverage 35.03% 35.03% ======================================== Files 31 31 Lines 12730 12730 ======================================== Hits 4460 4460 Misses 8270 8270 ``` | Flag | Coverage Δ | | |---|---|---| | unittests | `35.03% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jonas+Gli%C3%9F#carryforward-flags-in-the-pull-request-comment) to find out more.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Do you have feedback about the report comment? Let us know in this issue.