try out guzzle theme for documentation

[ctb]

Our old sphinx theme is kind of outdated looking ;). What do y'all think?

• Is it mergeable?
• make test Did it pass the tests?
• make clean diff-cover If it introduces new functionality in
  scripts/ is it tested?
• make format diff_pylint_report cppcheck doc pydocstyle Is it well
  formatted?
• Did it change the command-line interface? Only backwards-compatible
  additions are allowed without a major version increment. Changing file
  formats also requires a major version number increment.
• For substantial changes or changes to the command-line interface, is it
  documented in CHANGELOG.md? See keepachangelog
  for more details.
• Was a spellchecker run on the source code and documentation after
  changes were made?
• Do the changes respect streaming IO? (Are they
  tested for streaming IO?)

[ctb]

See docs here: https://khmer.readthedocs.io/en/docs-guzzle_theme/

[codecov-io]

Codecov Report

Merging #1630 into master will decrease coverage by -0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1630      +/-   ##
==========================================
- Coverage   69.81%   69.79%   -0.02%     
==========================================
  Files          66       66              
  Lines        8971     8970       -1     
  Branches     3060     3060              
==========================================
- Hits         6263     6261       -2     
- Misses       1025     1026       +1     
  Partials     1683     1683

Impacted Files	Coverage Δ
lib/storage.hh	89.28% <0%> (-0.52%)	❌
lib/hashtable.cc	55.18% <0%> (-0.22%)	❌

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[betatim]

Looks shiny ✨ Fine with me

[standage]

I approve this message.

[betatim]

Shiny new docs should appear soon.

[ctb]

Build failed: https://readthedocs.org/projects/khmer/builds/5100183/

[betatim]

on it