Migrate to django 3.2 for docs

[johannaengland]

As noticed in #2396 nav runs on django 3.2, but the docs use django 2.2.
This changes it so that #2396 works.

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
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[codecov]

Codecov Report

Merging #2399 (b995d09) into master (83a7835) will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2399      +/-   ##
==========================================
- Coverage   52.55%   52.51%   -0.04%     
==========================================
  Files         552      552              
  Lines       40152    40152              
==========================================
- Hits        21100    21087      -13     
- Misses      19052    19065      +13     

Impacted Files	Coverage Δ
python/nav/ipdevpoll/pool.py	84.88% <0.00%> (-5.04%)	⬇️

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[github-actions]

Test results

     12 files       12 suites   10m 54s ⏱️
3 077 tests 2 981 ✔️   96 💤 0 ❌
8 706 runs  8 418 ✔️ 288 💤 0 ❌

Results for commit b995d09.

[lunkwill42]

Good catch! :)