Update documentation and small Pixi improvements

[hoxbro]

Follow up to #6182, where we must align the rest of the documentation with the changes. I will also update these files to markdown.

• https://holoviews.org/user_guide/Installing_and_Configuring.html
• https://holoviews.org/install.html
• https://holoviews.org/index.html

It's not directly related, but I also want to update the changelog to only change once. I also removed .gitattribute as it is no longer needed.

I will also update this PR based on things I do it the Panel PR.
Todo based on work on panel

• Coverage trick
• Install only when needed on build step
• Make it so you can run the unit test without additional dependencies

[codecov]

Codecov Report

Attention: Patch coverage is 73.33333% with 24 lines in your changes are missing coverage. Please review.

Project coverage is 88.22%. Comparing base (8507aab) to head (f98ef05).
Report is 3 commits behind head on main.

Files	Patch %	Lines
holoviews/tests/core/test_options.py	62.96%	10 Missing ⚠️
holoviews/tests/operation/test_operation.py	63.63%	4 Missing ⚠️
holoviews/tests/util/test_utils.py	55.55%	4 Missing ⚠️
holoviews/tests/core/test_storeoptions.py	62.50%	3 Missing ⚠️
holoviews/tests/test_selection.py	60.00%	2 Missing ⚠️
holoviews/tests/conftest.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6200      +/-   ##
==========================================
+ Coverage   88.10%   88.22%   +0.12%     
==========================================
  Files         319      321       +2     
  Lines       67158    67285     +127     
==========================================
+ Hits        59168    59361     +193     
+ Misses       7990     7924      -66     

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[hoxbro]

Not sure if this is actually needed.

[hoxbro]

I will merge this as is. Again nothing is set in stone.

(This will also fix the current failing CI)

[github-actions]

This pull request has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.