Fix installation guide link in build_maxtext.md

[lydhr]

Description

Start with a short description of what the PR does and how this is a change from
the past.

The rest of the description includes relevant details and context, examples:

• why is this change being made,
• the problem being solved and any relevant context,
• why this is a good solution,
• some information about the specific implementation,
• shortcomings of the solution and possible future improvements.

If the change fixes a bug or a Github issue, please include a link, e.g.,:
FIXES: b/123456
FIXES: #123456

You can also provide a comma-separated list. If you don't want to close a bug but
simply to reference it, use BUGS, e.g.:
BUGS: b/123456

Notice 1: Once all tests pass, the "pull ready" label will automatically be assigned.
This label is used for administrative purposes. Please do not add it manually.

Notice 2: For external contributions, our settings currently require an approval from a MaxText maintainer to trigger CI tests.

Tests

Please describe how you tested this change, and include any instructions and/or
commands to reproduce.

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.

[gemini-code-assist]

Code Review

This pull request updates a relative link in the installation guide documentation to an absolute ReadTheDocs URL. The reviewer pointed out that hardcoding a specific version (maxtext-v0.2.3) in the URL makes it brittle and suggested using 'latest' or 'stable' instead to ensure the link remains up to date.

[gemini-code-assist]

Hardcoding a specific version (maxtext-v0.2.3) in the documentation URL makes it brittle and prone to becoming outdated when new versions are released. Consider using latest or stable in the ReadTheDocs URL to ensure the link always points to the most current version of the installation guide.

For example:
https://maxtext.readthedocs.io/en/latest/install_maxtext.html