docs: expand v5→v6 migration guide with all tracked breaking changes

[Copilot]

The existing migration page was a 22-line stub. This rewrites it into a complete guide covering every item in go-git/go-git#910.

Structure

• Status legend — ✅ Merged / 🔲 Planned / 💬 In Discussion on every section
• Quick checklist — 9 actionable items for a rapid codebase audit before upgrading
• Breaking changes (8 sections, each with what/why/impact + before/after code):
  • Repository.Close() now required (PR #1906)
  • git.PlainClone isBare positional param removed → CloneOptions.Bare
  • Tags no longer fetched by default on Fetch (PR #1459)
  • plumbing.TagMode → git.TagMode
  • config.Version_0/1 → config.Version0/1 (PR #1771)
  • commitgraph.Index gained io.Closer + new methods (PR #854)
  • osfs.BoundOS is now the default for Plain* operations
  • branch.*.merge config validation relaxed (PR #1923)
• Behaviour changes — DiffTreeWithOptions deprecation, Worktree.Add/AddGlob deprecation, Validate signature change
• Planned/TBD — SHA-256 (#706, #899), global git config (#395), new transport API, further repository.go changes
• Module path section and consolidated references

Also adds .gitignore to exclude the site/ mkdocs build artefact directory.

Original prompt

Create documentation in the go-git/docs repository that details behavior and API changes between go-git v5.x and the upcoming v6.0.0 major release.

Context / source of truth:

• Use go-git/go-git issue #910 (“API and Behaviour Changes for next major release”) as the canonical list of potential breaking or behavior changes and references to related issues/PRs.
• The doc should cite and link to the referenced issues/PRs and relevant code locations listed in #910.

Goals:

1. Add a new documentation page (or update the appropriate existing versioning / changelog / migration section) in go-git/docs that provides:
   • A high-level summary of what’s changing from v5 to v6.
   • A migration guide / checklist for users upgrading.
   • Sections for each breaking change and behavior change listed in #910, including:
     • What changed.
     • Why it changed (brief rationale if discoverable from linked issue/PR discussion).
     • Impact on users.
     • How to migrate / recommended code changes.
     • Links to the relevant go-git/go-git issue(s)/PR(s) and any referenced code lines.
   • Clear labels for status/certainty (e.g., “Planned”, “Merged in main”, “In discussion”, “TBD”), since some items are unchecked in #910.
2. Ensure the documentation is easy to find from the docs navigation (add to sidebar/nav if needed).
3. Use consistent formatting and style with existing docs.

Implementation details:

• Inspect the existing structure of go-git/docs to determine where release notes/migration guides live.
• Prefer a file name like migrating-to-v6.md or v6-migration-guide.md (or whatever matches repo conventions).
• Include links to:
  • go-git/go-git issue #910
  • Cross-referenced issues (#706, #899, #395, #1923, etc.)
  • PRs mentioned (#854, #1459, #1771, #1906, etc.)
  • Any referenced code locations (permalinks are fine).

Acceptance criteria:

• A PR is opened against go-git/docs with the new/updated documentation.
• The page clearly distinguishes breaking changes vs behavior changes.
• Every item in #910 appears in the docs page (even if marked TBD/Planned), with at least a short description and link to its tracking issue/PR.
• Docs build (if applicable) passes.

Notes:

• The user request: “Write a PR on go-git/docs detailing the behaviour/api changes between V5 and v6. Please refer to @go-git/go-git/issues/910 for all the references of issues and PRs that may have some level of breaking change.”

The following is the prior conversation context from the user's chat exploration (may be truncated):

User: Write a PR on go-git/docs detailing the behaviour/api changes between V5 and v6.

Please refer to @go-git/go-git/issues/910 for all the references of issues and PRs that may have some level of breaking change.

Assistant: A pull request creation has been prepared and is waiting on your approval in the confirmation dialog—once you accept it, I can proceed to generate and open the PR in go-git/docs with a v5→v6 migration/behavior-change guide based on go-git/go-git issue #910.

This pull request was created from Copilot chat.