Write about Tier 2 and JIT in "what's new 3.13"

[gvanrossum]

Here's some text for "what's new 3.13" describing the Tier 2 and JIT architecture. It's not complete and missing references and details -- we can always improve it later, but suggestions for improvements right now are also welcome.

I used "JIT" as the headline, rather than "Tier 2" or "micro-ops", mostly because that's apparently the catchier phrase, and also because the JIT gives the best performance. (Numbers are TBD -- they will improve over the next few months as more of the optimization pipeline lands.)

It's a bit speculative because gh-114059 (@Fidget-Spinner's optimizer) hasn't landed yet -- we can wait merging this until it has.

Once this is in, I am hoping (based on an email exchange with @Yhg1s) that this work can also be highlighted in the alpha release announcements.

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📚 Documentation preview 📚: https://cpython-previews--114826.org.readthedocs.build/

[Fidget-Spinner]

There are in general two editorial directions we can take:

1. Go into a bit more technical details, like what I chose to do in 3.11 https://docs.python.org/3/whatsnew/3.11.html#faster-cpython
2. Leave the technical details out and give a high-level overview of the work. Write a blog post somewhere else to detail the technical stuff.

Personally, I am biased and in favour of 1. :). Anecdotally, it was quite well received among tech communities. People want to know exactly what it is that we improved, and the docs often form the authoritative source of information, and perhaps the only source of information some people know. I recall the 3.11 Faster CPython section being shared multiple times on various tech forums, and tech news sites.

Let me know what you think. I would love to collaborate on this. I like technical writing, just haven't had the time to do much recently.

[Fidget-Spinner]

I created gh-114863 to track whats new changes for faster cpython 3.13. Also #114862 as this PR but added more stuff.

[gvanrossum]

@Fidget-Spinner I am fine with you writing a longer version. I've merged this one so I get partial credit. :-)