docs: Clarify `CreateTree` semantics and `TreeEntry` usage

[rogpeppe]

Summary

This PR improves the documentation for CreateTree and TreeEntry to clarify several behaviors that are not obvious from the existing docs:

• Merge semantics: When baseTree is provided, entries are merged with that tree - paths not mentioned in entries are preserved from the base tree
• Duplicate entries: If the same path appears multiple times in entries, the last entry wins
• File/directory deletion: To delete an entry, set both SHA and Content to nil; this works for both files and entire directories
• Type/Mode ignored on delete: When deleting, the Type and Mode fields are ignored by the API; only Path is required
• Content vs SHA: Clarifies when to use Content (small text files) vs SHA (reference to existing blob), with a reference to CreateBlob for binary files

These behaviors were verified experimentally against the GitHub API.

Test plan

• [x] Documentation-only change, no code changes
• [x] Existing tests pass (go test ./github/... -run TestGitService_.*Tree)

🤖 Generated with Claude Code

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 92.48%. Comparing base (59f33d9) to head (d61fb68). :warning: Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3877   +/-   ##
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  Coverage   92.48%   92.48%           
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  Files         200      200           
  Lines       14564    14564           
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  Hits        13469    13469           
  Misses        895      895           
  Partials      200      200           

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[gmlewis]

Thank you, @stevehipwell! Merging.