doc: add a Git guide and fix addconsoletrace typo

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>

common/utils/LOG/DOC/addconsoletrace.md

@@ -11,7 +11,7 @@ LOG_D(<component>,<format>,<argument>,...)
 LOG_T(<component>,<format>,<argument>,...)
 )
 ```
-these macros are used in place of the printf C function. The additionnal ***component*** parameter identifies the functionnal module which generates the message. At run time, the message will only be printed if the configured log level for the component is greater or equal than the macro level used in the code.
+these macros are used in place of the printf C function. The additional ***component*** parameter identifies the functional module which generates the message. At run time, the message will only be printed if the configured log level for the component is greater or equal than the macro level used in the code.
 
 | macro | level letter | level value | level name |
 |:---------|:---------------|:---------------|----------------:|