Improve documentation theme

[Bultako]

This PR addresses most of the feedback gathered in #3746:

• style more similar to https://gammapy.org/
• changelog refactored into individual RST pages
• moved Dependencies to Development section
• link to recipes as a panel box in Userguide
• link to tutorials in Useful links in home page
• text modified and reduced on homepage
• add acknowledge link in the nav bar
• add slack icon in the bav bar
• fix warnings in docstrings
• order models in API
• add version switching select box in the nav bar

Still some modifs missing and to be done in another PR:

• modify the text and style of HowTo panels
• refactor and move text in API ref index pages

You may access how the docs shows with this PR in the usual link.

[codecov]

Codecov Report

Merging #3817 (d5aa90a) into master (1c043d3) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3817   +/-   ##
=======================================
  Coverage   93.83%   93.83%           
=======================================
  Files         162      162           
  Lines       19778    19778           
=======================================
  Hits        18558    18558           
  Misses       1220     1220           

Impacted Files	Coverage Δ
gammapy/data/data_store.py	90.87% <ø> (ø)
gammapy/data/event_list.py	94.72% <ø> (ø)
gammapy/datasets/map.py	93.25% <ø> (ø)
gammapy/estimators/points/profile.py	90.47% <ø> (ø)
gammapy/estimators/utils.py	90.90% <ø> (ø)
gammapy/irf/core.py	89.35% <ø> (ø)
gammapy/irf/edisp/kernel.py	98.47% <ø> (ø)
gammapy/irf/psf/parametric.py	92.48% <ø> (ø)
gammapy/makers/background/reflected.py	97.79% <ø> (ø)
gammapy/makers/background/ring.py	97.05% <ø> (ø)
... and 9 more

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[bkhelifi]

Thanks a lot Jose Enrique, looks nice...
A fast question: have we decided if we promote a bit the use of mamba? Because in the 'getting started' it is not mentioned

[Bultako]

@bkhelifi
well, it is actually mentioned https://docs.gammapy.org/dev/search.html?q=mamba
though maybe not in a prominent way..

[bkhelifi]

In https://www.iaa.csic.es/~jer/gammapydocs/userguide/index.html,

• the recipes do not appear in the left column...
• Should we not make 2 boxes for the first 2 items (gamma-ray analysis and package structure)? An open question...

[Bultako]

the recipes do not appear in the left column...

Done

I also added the slack icon and one acknowledge link in the nav bar.
This is now ready for review.

[adonath]

Thanks a lot @Bultako! For now I only have two minor comments:

• The fontsize in the navbar could a bit larger
• The version switch would be great
  Otherwise I'd be happy to merge this any time...

[registerrier]

Thanks @Bultako . This looks really great! I have only one comment.

Splitting the changelog reads very well. That's a very good idea.
But isn't it an issue if CHANGES.rst is removed from the main directory?
We could keep it even if the content is duplicated between the general directory and the docs, no?

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[Bultako]

@adonath @registerrier

Your last comments have been addressed.

• fontsize in the navbar a bit larger
• version switcher implemented
• recover CHANGES.rst (event if we have duplicated content, I agree it is good to have it at the root level of the repo, at least for quick inspection in github)
• amend text on how to build docs as a pdf file

You may have a quick look at the usual link.
Otherwise, ready to merge from my side.

[adonath]

Thanks a lot @Bultako, nor further comments from my side. Later I guess we could just implement a little helper script, that copies the changes into single CHANGES.rst.

[adonath]

I'd love to see this online. So I'll go ahead and merge this. We can always implement changes and improvements in follow up PRs.

[adonath]

Thanks a lot @Bultako!