[MRG] add information about versioning, migrations, etc to the docs

[ctb]

Upgrade documentation in preparation for v4.

ref #1033, #655, #1069, #1145, #1128

• Is it mergeable?
• make test Did it pass the tests?
• make coverage Is the new code covered?
• Did it change the command-line interface? Only additions are allowed
  without a major version increment. Changing file formats also requires a
  major version number increment.
• Was a spellchecker run on the source code and documentation after
  changes were made?

[ctb]

@luizirber the overall shape of the text is at a resting place, and I welcome your thoughts on what topics I'm missing! (I've yet to review the actual formatting.)

[codecov]

Codecov Report

Merging #1153 into latest will increase coverage by 9.14%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           latest    #1153      +/-   ##
==========================================
+ Coverage   83.72%   92.86%   +9.14%     
==========================================
  Files          95       71      -24     
  Lines        9012     5636    -3376     
==========================================
- Hits         7545     5234    -2311     
+ Misses       1467      402    -1065     

Flag	Coverage Δ
#rusttests	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
sourmash/signature.py	92.53% <0.00%> (-0.29%)	⬇️
sourmash/sbtmh.py	89.52% <0.00%> (-0.20%)	⬇️
sourmash/command_compute.py	97.43% <0.00%> (-0.20%)	⬇️
sourmash/nodegraph.py	93.45% <0.00%> (-0.18%)	⬇️
sourmash/minhash.py	97.10% <0.00%> (-0.16%)	⬇️
sourmash/sbt_storage.py	92.42% <0.00%> (-0.12%)	⬇️
sourmash/index.py	97.56% <0.00%> (-0.09%)	⬇️
sourmash/sbt.py	88.49% <0.00%> (-0.07%)	⬇️
sourmash/lca/lca_db.py	97.16% <0.00%> (-0.05%)	⬇️
sourmash/cli/__init__.py	98.90% <0.00%> (-0.02%)	⬇️
... and 24 more

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[luizirber]

@luizirber the overall shape of the text is at a resting place, and I welcome your thoughts on what topics I'm missing! (I've yet to review the actual formatting.)

I'm pretty happy with it as-is, I think it touches all the important points!

[ctb]

Ready for review!