Add doc/git-guide.md, a single entry point for the practical Git
knowledge needed to contribute to OAI, consolidating the how-tos
that were scattered across the documentation:

- Setting up commit signing: moved from CONTRIBUTING.md, which keeps
  the normative DCO/Verified requirements and links to the guide; led
  by a compact SSH recipe, with the allowed_signers troubleshooting
  and a note on repository-level configuration for shared servers.
- Managing your own branch: moved from code-style-contrib.md, which
  keeps the workflow policy and links to the guide; mentions git
  switch and covers fixup commits with git rebase --autosquash,
  including the --fixup=amend:<commit> variant.
- Working with submodules: the update command and the unintended
  submodule-pointer-update pitfall seen in past PRs.
- Recovering from mistakes: git restore --staged, git restore,
  git reset --soft/--hard, and git reflog.
- Reusing conflict resolutions with git rerere: enabling, typical
  flow, inspecting, seeding from existing history, sharing the cache,
  and caveats, clarifying that contribution branches must keep a
  linear history while forks may carry merge commits.

The guide is referenced from doc/README.md (Developer tools),
code-style-contrib.md, and CONTRIBUTING.md. Guidance coupled to its
own document is cross-linked instead of moved: clang-format.md
(pre-commit hook), GET_SOURCES.md (branch and tag model), and
doc_best_practices.md (docs: commit prefix).

Also fix a typo in the T tracer addconsoletrace documentation.
Signed-off-by: Andrea Lacava <thecave003@gmail.com>