Include PyNumero "inventory" in readme

[Robbybp]

Summary/Motivation:

PyNumero is a fairly frequently used contrib package, so we would like to keep it as stable as possible. However, there is also a lot of experimental code in PyNumero. This PR attempts to distinguish between the "core functionality" of PyNumero and other experimental features.

Changes proposed in this PR:

• Explicitly mention which parts of PyNumero are stable in the online docs
• Add an "inventory" to the readme, which attempts to classify all of PyNumero's code as either stable or experimental

Legal Acknowledgement

By contributing to this software project, I have read the contribution guide and agree to the following terms and conditions for my contribution:

1. I agree my contributions are submitted under the BSD license.
2. I represent I am authorized to make the contributions and grant the license. If my employer has rights to intellectual property that includes these contributions, I represent that I have received permission to make contributions and grant the required license on behalf of that employer.

[michaelbynum]

Very nice!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.40%. Comparing base (ed6296b) to head (5689e74).
Report is 75 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3248      +/-   ##
==========================================
- Coverage   88.40%   88.40%   -0.01%     
==========================================
  Files         849      849              
  Lines       95343    95343              
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- Hits        84289    84288       -1     
- Misses      11054    11055       +1     

Flag	Coverage Δ
linux	86.36% <ø> (ø)
osx	76.21% <ø> (ø)
other	86.55% <ø> (-0.01%)	⬇️
win	83.85% <ø> (-0.01%)	⬇️

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