Fix deprecated sphinx html_context usage in conf.py

[djhoese]

We were using the old html_context which has been deprecated for a while. This PR switches to html_css_files instead. See sphinx-doc/sphinx#8885 for more information.

[djhoese]

@mraspaud This fixes the website building and should be squash merged. I couldn't stand that I had mislabeled the step for building the website as "Run unit tests" (copy/paste error) so now there are two commits. 😢

[codecov]

Codecov Report

Merging #1545 (0ce1d0a) into master (b792f29) will decrease coverage by 1.13%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1545      +/-   ##
==========================================
- Coverage   92.09%   90.95%   -1.14%     
==========================================
  Files         251      251              
  Lines       36711    36711              
==========================================
- Hits        33808    33391     -417     
- Misses       2903     3320     +417     

Flag	Coverage Δ
behaviourtests	4.48% <ø> (ø)
unittests	91.47% <ø> (-1.15%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
satpy/tests/reader_tests/test_seviri_l2_grib.py	23.07% <0.00%> (-76.93%)	⬇️
satpy/tests/reader_tests/test_seviri_l2_bufr.py	30.35% <0.00%> (-69.65%)	⬇️
satpy/readers/iasi_l2_so2_bufr.py	25.80% <0.00%> (-68.82%)	⬇️
satpy/readers/ascat_l2_soilmoisture_bufr.py	29.87% <0.00%> (-67.54%)	⬇️
satpy/tests/reader_tests/test_iasi_l2_so2_bufr.py	30.98% <0.00%> (-66.20%)	⬇️
...ts/reader_tests/test_ascat_l2_soilmoisture_bufr.py	37.20% <0.00%> (-60.47%)	⬇️
satpy/readers/seviri_l2_grib.py	26.53% <0.00%> (-60.21%)	⬇️
satpy/readers/seviri_l2_bufr.py	34.52% <0.00%> (-59.53%)	⬇️
satpy/node.py	87.96% <0.00%> (-0.93%)	⬇️
satpy/dependency_tree.py	97.23% <0.00%> (-0.35%)	⬇️
... and 2 more

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[mraspaud]

LGTM