DOC: add a pre-commit hook to spell-check markdown files in docs/

[ev-br]

Description

Add a pre-commit hook for spell-checking markdown docs in the docs/ folder.

When typo's found, interactively prompt the user whether to block the commit, ignore the word, or add it to the project-specific dictionary.

Spell-checking itself relies on GNU Aspell utility, http://aspell.net/, which needs to be installed separately.

The wrapper is minimally modified from https://github.com/mprpic/git-spell-check to be used with the pre-commit framework.
The original version is GPL licensed, and this one is clearly a derivative.
Please advise if this is a problem. If it is, one solution could be to package this hook as a separate repository instead of a local hook. That way, maxtext repository will only carry the hook configuration, which is clearly not copyrightable.

Checklist

Before submitting this PR, please make sure (put X in square brackets):

• I have performed a self-review of my code. For an optional AI review, add the gemini-review label.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run end-to-end tests tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed, including adding new documentation pages to the relevant Table of Contents (toctree directive) as explained in our documentation.