Allow to pass a dict for the summarized fields, fix the doc rendering

[m-mohr]

Related Issue(s):

• None

Description:

This implements two changes:

• Allow to pass a Dict with field names and summary strategies. Some people didn't realize they could set the summaryfields property so this makes it more obvious that you don't necessarily need to create a JSON file.
• Fix the foc rendering:
  
• Generally improve the summaries section a bit based on questions that I was asked during consulting.

PR Checklist:

• pre-commit hooks pass locally
• Tests pass (run scripts/test)
• Documentation has been updated to reflect changes, if applicable
• This PR maintains or improves overall codebase code coverage.
• Changes are added to the CHANGELOG. See the docs for information about adding to the changelog.

[codecov]

Codecov Report

Patch coverage: 92.85% and no project coverage change.

Comparison is base (ebe5c6b) 92.01% compared to head (a9d016f) 92.01%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1195   +/-   ##
=======================================
  Coverage   92.01%   92.01%           
=======================================
  Files          51       51           
  Lines        6809     6813    +4     
  Branches     1007     1009    +2     
=======================================
+ Hits         6265     6269    +4     
  Misses        368      368           
  Partials      176      176           

Files Changed	Coverage Δ
pystac/summaries.py	77.77% <92.85%> (+0.48%)	⬆️

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[jsignell]

This looks good to me! Nice clean up 👍