gh-113664: Improve style of Big O notation (math)

[serhiy-storchaka]

Use :math: to render them as mathematic formulas.

• Issue: bisect.insort*() docs: inconsistent formatting of O()  #113664

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📚 Documentation preview 📚: https://cpython-previews--113724.org.readthedocs.build/

[hugovk]

Does :math: also work with PDF, EPUB and plain text builds?

https://docs.python.org/3/download.html

[erlend-aasland]

Digression: the Big O notation in select.rst is ... untraditional. I've never seen O(n) written as O(name of some variable). IIRC, all off these should be written as O(n). One can of course clarify n in a follow-up sentence in the docs.

[serhiy-storchaka]

I cannot build EPUB due to unrelated error.

In PDF it renders naturally, except cases in select.rst where spaces between words are swallowed: $O(highest file descriptor)$. I tried different variants, like \text{} or \mathrm{}, but they do not work in HTML. Perhaps the best way is to simply use italic font here.

In plain text it is rendered literally, e.g. O(n\log{n}).

[hugovk]

This PR with EPUB (in macOS Books):