docs: add release branching documentation

[jescalada]

Closes #1483.

The updated workflows have been added in #1520, all documentation here reflects the changes included in that PR.

We should decide which versions will be supported (with occasional bugfixes). I thought about supporting 1.20 and 2.x, and then dropping support for 1.20 when 3.0 comes out, but v1 has drifted so much from 2.0 that we'd be better off dropping support entirely to avoid all the extra work. Either way, this doc should be updated to inform users/maintainers about which GitProxy versions have active support.

Notes

• I wrote this with the assumption that we'll be doing cascade-based merges rather than cherry-picking, as @coopernetes suggested in Enforce GitLabFlow branching strategy #1483
  • For reference, some pros and cons of cascade merges
• We might want to add a section to explain which versions will be supported, in order to limit the amount of extra work and merge conflicts when cascading bugfixes. For example, only the latest of each major version (currently 1.19 and 2.0)
• I mentioned that releases are supposed to follow a cadence, but I never specified what that cadence should be. We should decide on an appropriate cadence (say, every three months) or remove that part entirely.

Would love to hear your thoughts! @finos/git-proxy-maintainers

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.21%. Comparing base (b6d72fc) to head (433c378).

Additional details and impacted files

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##             main    #1516   +/-   ##
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  Coverage   90.21%   90.21%           
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  Files          69       69           
  Lines        5511     5511           
  Branches      944      944           
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  Hits         4972     4972           
  Misses        521      521           
  Partials       18       18           

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