Enable codespell on book docs to catch typos/misspellings

[philrz]

Now that some known typos/nits in the book docs have been addressed (#7234, #7236, #7242) here a Makefile target is added to invoke codespell so we'll get an early notice locally and in CI if new mistakes are made in future book edits, rather than catching each other's mistakes manually in PR review as we've done in the past.

This takes < ~0.4 seconds to run locally on my Intel Macbook, so it comes cheap.

Here's an example of what it looks like flagging an intentional mistake:

$ make -C book codespell
git ls-files -c -o --exclude-standard | xargs codespell -S src/tutorials/prs.json
src/intro.md:22: teh ==> the
make: *** [codespell] Error 1

[nwt]

Any reason to use pipx instead of pip?

[philrz]

@nwt: The Python people cite it at https://packaging.python.org/en/latest/guides/installing-stand-alone-command-line-tools/ as a best practice, but I've confirmed that the GitHub Actions Linux Runners would still run this fine with regular pip. Would you prefer I make that change before I merge this?

[nwt]

Yeah, might as well.