[Feature]: last-edited dates on instruction pages

### Feature Description

on a page like https://intel.github.io/SDM/sdm.html?instr=ADDSS, it would be great to have a date for the last time the content on the page was substantially changed. CSS changes of course are probably not interesting, fixing a typo might be, and fixing a bug in an instruction's description probably is!

### Why Is This Needed?

`intel.github.io` is a more authoritative source than something like (the loved and appreciated!) docs at https://www.felixcloutier.com/x86/xadd - I know Félix's site is best effort and every page says as much in the footer. if I'm doing something _interesting_ I historically might have started at that site, fetched a fresh copy of the SDM, and noted which revision of the SDM I was working from.

importantly, I may quote parts of the SDM as explanatory prose in a system I'm working on, and I include the version of the SDM I'm working to help later readers find (or explain the lack of) the quoted passage. quoting `intel.github.io` is different, since clearly it is not a document one can download, but it _is_ a git repo one can easily fetch the history of. thinking through how I would quote `intel.github.io`, I would probably include a note like "as of Sept 4, 2026", and know that later I should check the page at that date and potentially update the date to the next time I've verified the docs match the reasoning I'm working with.

a "last updated" date on the page itself helps short-circuit all of this: a git edit date does not immediately indicate if the substantive content has changed in the same way an intentional date describing the encoding rules, description, operation, exceptions, etc would. at that point I wouldn't feel a need to cite either a specific rev of `github.com/intel/SDM` or corresponding SDM version. in the ideal world, and tragically, I'll stop having an accurate collection of every SDM revision.... alas.

I realize maintaining such a date would be a little complicated, and I don't have great ideas there, sorry! :)

Start with the instruction page at https://intel.github.io/SDM/sdm.html?instr=ADDSS and inspect how its content is generated from the intel/SDM repository. Determine how substantive documentation changes can be distinguished from CSS edits and typo fixes, then define how the resulting date should appear on each page. Done means instruction pages show a meaningful last-edited date that reflects substantive content changes.

Written by the indexing model from the issue text.