Upgrade to MathJax v3 (replaces retired cdn.mathjax.org)

[aoboturov]

Summary

The site loaded MathJax from //cdn.mathjax.org/..., retired in 2017, so all math on the rendered notes (e.g. the autoregressive page) was broken — symbols like \mathcal{D} showed up as tofu boxes.

This PR migrates from MathJax v2 (dead CDN) to MathJax v3 on jsDelivr. v3 was chosen over a v2-on-jsDelivr swap because v2's HTML-CSS renderer was failing to load its calligraphic webfont in testing, and v3 ships fonts inline in a single bundle, which is more robust.

Changes

• Replace the dead cdn.mathjax.org script tag with https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-mml-chtml.js.
• Move the MathJax config from _layouts/post.html into _includes/head.html, since v3 requires window.MathJax = {...} to be set before the loader runs.
• Convert the v2 MathJax.Hub.Config({ TeX: { Macros: ... } }) syntax to v3 window.MathJax = { tex: { macros: ... } }. All custom macros (\Dc, \Exp, \KL, \note, etc.) are preserved.
• Explicitly enable $...$ / $$...$$ delimiters — v3 defaults to only \(...\) / \[...\], but the source markdown uses $.
• Add a startup.pageReady shim that converts kramdown's legacy <script type="math/tex">...</script> wrappers into \(...\) / \[...\] before typesetting. v3 doesn't recognise the v2 script-tag protocol, and the built HTML in docs/ already contains those wrappers.
• Load the TeX color package so \textcolor{...} (used in the \note macro) keeps working.
• Apply the same edits to the pre-built docs/**/index.html files so the fix takes effect on GitHub Pages without a Jekyll rebuild (_config.yml has destination: docs).

Test plan

• Open any notes page (e.g. /notes/autoregressive/) — \mathcal{D}, n, \mathbf{x}, display equations, and macros all render properly.
• Browser console shows no cdn.mathjax.org has been retired warning and no MathJax errors.
• Verified locally by serving docs/ under the /notes baseurl and confirming math renders end-to-end.

[aoboturov]

Live preview of this branch: https://aoboturov.github.io/notes/autoregressive/ (and other notes paths). Compare against the broken https://deepgenerativemodels.github.io/notes/autoregressive/ — the calligraphic $\mathcal{D}$, inline italics, and display equations should all render here.