DOC: fix typos in documentation

[MTerwissen]

Closes #595

📝 Documentation

• Fixed typos, doubled words, and grammar in docs/usage.ipynb, docs/usage/basics.ipynb, docs/usage/caching.ipynb, docs/usage/faster-lambdify.ipynb, and docs/amplitude-analysis/analytic-continuation.ipynb. Most notably, the faster-lambdify page now says that sub-expressions are lambdified separately and then recombined, and the second caveat in the hit-and-miss warning of docs/usage/basics.ipynb now reads as a sentence ("If the function displays narrow structures, […] chances are smaller that […]").
• Fixed further spelling and grammar mistakes in docs/amplitude-analysis.ipynb, docs/usage/chi-squared.ipynb, docs/usage/unbinned-fit.ipynb, and docs/install.md, such as "the helicity formalism boosts", "simplify the expression", "an UnbinnedNLL", and "To get JAX and TensorFlow to work".
• Fixed the spelling of "expression" in the docstring of create_cached_function().

🖱️ Developer experience

• Disabled allowCompoundWords in .cspell.json, so that misspellings can no longer pass as concatenations of valid words. Legitimate technical terms that were previously accepted as compounds are now listed in the cSpell dictionary.

🔨 Maintenance

• Renamed the private function get_jit_compile_dectorator() to get_jit_compile_decorator() and fixed a misspelled local variable in the tests.

Squash commit messages

* DX: disallow compound words in cspell
* MAINT: fix typos in private function names

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[redeboer]

Thanks, LGTM! 🦒