Add daily warning message about ArviZ's refactoring

[aloctavodia]

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📚 Documentation preview 📚: https://arviz--2485.org.readthedocs.build/en/2485/

[OriolAbril]

Now that I am seeing this. Do we want to include this in the next release or first make a release without warnings yet but including the fix for i/o, attrs in idata/datatree conversion... then make a follow up release to add the warning?

I think the next release is the one that will allow proper migration through the arviz.preview and I think it would be nice to have a version where users can actively work on the migration without warnings yet. The migration is bound to take some time.

On the other hand, if we add a pin for the "loud" version of arviz in downstream dependencies like pymc then downstream libraries will know to migrate but users won't which is also not a good development

[tomicapretto]

TIL about read_text and write_text!

[tomicapretto]

@aloctavodia approved after you apply @OriolAbril's suggestion

[aloctavodia]

We should have a migration plan. That may be something like

1. Do a new release of arviz-xyz libraries.
2. Do a new release of arviz, including the warning message.
3. Help PyMC/Bambi (and potentially others) with the transition. For instance, we could start adapting examples (but probably also changing internal where arviz is used).

We may want to have some tentative dates for these steps, even if vague and subject to change. We may want to add more steps, too.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.82%. Comparing base (fae2e4d) to head (e33c4f6).
⚠️ Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2485      +/-   ##
==========================================
- Coverage   86.83%   86.82%   -0.01%     
==========================================
  Files         124      124              
  Lines       13018    13035      +17     
==========================================
+ Hits        11304    11318      +14     
- Misses       1714     1717       +3     

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