docs: clarify & promote version APIs

[jpnurmi]

Document and clarify the version macros (#2136) that were intended for downstream SDKs but are useful for external users, too. Take the opportunity to expand the related sentry_sdk_version docs and drop the experimental stamp, as it's been there for 4+ years (introduced in June 2022: b091b76).

Ref: #2116

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.48%. Comparing base (a22b118) to head (f2a60ae).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2176   +/-   ##
=======================================
  Coverage   75.47%   75.48%           
=======================================
  Files         103      103           
  Lines       28079    28079           
  Branches     5147     5147           
=======================================
+ Hits        21194    21196    +2     
- Misses       5538     5542    +4     
+ Partials     1347     1341    -6     

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