doc: Enable MathJax rendering for equations in MkDocs documentation

[gulshan-123]

Solves #6610, Hi I am a new contributor to grass! Excited to contribute more...

This PR enables proper rendering of mathematical expressions in the GRASS MkDocs documentation.
Some manual pages such as r.runoff contain LaTeX formulas, but these were previously shown as raw $...$ text. This update integrates MathJax and the required Markdown extension so that equations render correctly both inline and in block form.

---

Testing

## Math test

Inline: $x^2 + y^2 = z^2$.

Block:

$$
E = mc^2
$$

Its Output:

---

Notes from local build experience

While testing this change locally, I found it non-obvious how to build and preview the documentation.
At first, addon manual pages like r.runoff were missing, and changes in index.md had no visible effect.
Eventually I realized that MkDocs serves content from the generated directory:

dist.x86_64-pc-linux-gnu/docs/mkdocs/

and not directly from files under man/.
After updating the correct mkdocs.yml and the generated source/index.md inside the dist folder, MathJax worked as expected.
The final changes in this PR are applied to man/mkdocs/mkdocs.yml

[gulshan-123]

I have also tested the r.runoff file by pasting it to index.md just for local testing.

Results is as required:

[petrasovaa]

Thanks! There is another PR (#6610) opened for this, but the author didn't follow up on a question we had there that applies to your PR as well

Why is the polyfill needed and have you tried using mathjax 4.0?

[gulshan-123]

Thank you for the review, I have now changed to v4.

Why is the polyfill needed and have you tried using mathjax 4.0?

polyfill was required to be used on some older browser for v3.
But, now I am using version 4, so polyfill is not required and not recommended.

Quoting from docs:
Browser Compatibility

MathJax supports all modern browsers (Chrome, Safari, Firefox, Edge), and most mobile browsers. We no longer test MathJax with IE11, so you should not expect it to work with any version of Internet Explorer. We used to recommend loading the polyfill library in order to help MathJax work with older browsers, but that is not necessary for modern browsers, and we no longer recommend it.

EDIT: I have tested the doc build, The result is the same, thus we should keep v4