Add Node 20 to the CI workflow, move building docs to Node 20

[Monteth]

Node 20 is a current Node release, and our newest version (Node 18) loses its LTS status in 4 months.

[codecov[bot]]

Codecov Report

Merging #1274 (cfda1ab) into master (1c69877) will decrease coverage by 1.86%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1274      +/-   ##
==========================================
- Coverage   96.51%   94.66%   -1.86%     
==========================================
  Files         231      231              
  Lines        3821     3821              
  Branches     1029     1029              
==========================================
- Hits         3688     3617      -71     
- Misses         12       82      +70     
- Partials      121      122       +1     

see 8 files with indirect coverage changes