Prettify the RTD pages

[adybbroe]

This PR seeks to update the presentation of the on-line documentation (the Readthedoc) pages to something more nicer looking. The pages are a bit old and are using an old (default) Sphinx theme.

Here is how the current pages looks for me in Firefox:

Would be nice to have something coherent across all Pytroll RTD pages, so copying configurations from Satpy's RTD documentation.

• [ ] Closes #xxxx
• [ ] Tests added
• [ ] Tests passed
• [ ] Passes flake8 pyorbital
• [ ] Fully documented

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (86bb368) 87.50% compared to head (4ccdde8) 87.50%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #137 +/- ## ======================================= Coverage 87.50% 87.50% ======================================= Files 14 14 Lines 2209 2209 ======================================= Hits 1933 1933 Misses 276 276 ``` | [Flag](https://app.codecov.io/gh/pytroll/pyorbital/pull/137/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/pytroll/pyorbital/pull/137/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll) | `87.50% <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=pytroll#carryforward-flags-in-the-pull-request-comment) to find out more.

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[coveralls]

coverage: 87.556%. remained the same when pulling 4ccdde8993e065738c1fa595974604ce7a565c08 on adybbroe:rtd_facelift into 86bb368bd3d0b2027bd35ecb5605cb34e6741b91 on pytroll:main.

[djhoese]

Were the Makefile changes caused by copying Makefile from Satpy to here? It looks like Satpy's Makefile (and therefore sphinx project) may have been older. I see a lot of build options removed.

[adybbroe]

Off-line and locally the pages look like this for me now: