docs: document configurable templates

[markphelps]

Summary

• Document server-wide configurable commit and merge proposal templates
• Add merge proposal examples and template context details
• Clarify repository-level templates override server-wide defaults and the current UI override limitation

Verification

• Verified implementation against flipt-io/flipt branch v2
• npm run lint -- --quiet v2/configuration/overview.mdx v2/guides/user/environments/merge-proposals.mdx
• npx prettier --write v2/configuration/overview.mdx v2/guides/user/environments/merge-proposals.mdx
• git diff --check
• Fresh reviewer pass

Closes #411

[claude]

Review

Overall this is a clear, well-structured addition — it follows the existing table/Note conventions in overview.mdx and gives both a server-wide and repository-level example in merge-proposals.mdx, matching the "include both basic and advanced use cases" guideline. A few things worth addressing before merge:

Possible issues

1. .Changes is used but never documented (docs/v2/configuration/overview.mdx:641)
   The commit_message example uses {{ (index .Changes 0) }}, but there's no context table describing .Changes anywhere — unlike proposal_title/proposal_body, which get a full field table in merge-proposals.mdx. Readers have no way to know what .Changes contains (a list of strings? structs?) or what other fields might be available for the commit message template. Consider adding a short context table for commit_message similar to the one for merge proposals, or at least a sentence explaining the shape of .Changes.

2. Behavior for invalid repository-level templates is unspecified
   The <Note> in overview.mdx:652-655 says invalid server-wide templates prevent configuration from loading, but neither file says what happens if a repository-level (flipt.yaml) template is invalid — does it fail the sync, fall back to the server-wide/default template, or error out only when a proposal is created? This seems like a meaningfully different failure mode worth documenting explicitly, especially since repo-level templates are edited by end users (not operators) and are more likely to contain typos.

3. Heading order breaks the existing alphabetical convention
   Under ## Additional Settings, the new ### Templates section is inserted before ### License and ### Meta & Diagnostics, but those two are already in alphabetical order. Alphabetically, Templates should come last. Minor, but worth a quick fix for consistency.

Nit

• {{ (index .Changes 0) }} — the outer parens aren't needed in Go's text/template syntax; {{ index .Changes 0 }} is the more idiomatic/common form and matches how .Base.Ref/.Branch.Ref are written elsewhere in the same PR without extra grouping.

Verification caveat
I wasn't able to reach flipt-io/flipt from this sandbox to independently confirm the config keys (templates.commit_message, templates.proposal_title, templates.proposal_body), the v2.5.0 "Since" version, or the exact template context field names (.Base, .Branch, .Ref, .Directory). The PR description says this was verified against the flipt-io/flipt v2 branch, which is good — just flagging that I couldn't independently re-verify it, and getting the field names right matters here since a typo would silently break user-authored templates rather than fail loudly.

What looks good

• Frontmatter unaffected (no new files, existing pages already have it).
• Correct markdown link syntax for the text/template reference per the style guide.
• <Note> callouts used appropriately to flag the UI override limitation and validation behavior.
• Table formatting matches the rest of overview.mdx (Property / Description / Default / Since).