docs: Make US spelling the default again

[izmalk]

Description of issue or feature:

The Vale spell checker was using a dictionary from the Canonical documentation style guide that contained both US and UK English spellings (81,536 entries). This meant UK spelling variants like "colour", "centre", "organise", "analyse", and "behaviour" were not flagged as errors, leading to inconsistent American English usage in the documentation.

Solution:

Replaced the mixed-language dictionary with a US-only SCOWL-based Hunspell dictionary (49,569 entries, version 2020.12.07) from wordlist.sourceforge.net (source). The dictionary files are now stored directly in the repository at .sphinx/dictionaries/ and copied to the Vale configuration during make spelling.

Changes made:

Added US-only dictionary files to .sphinx/dictionaries/ (en_US.dic, en_US.aff)
Updated Makefile vale-install target to copy local dictionary files instead of downloading
Added a comprehensive README.md documenting the dictionary source, usage, and verification
UK spellings are now properly flagged as spelling errors, while US spellings are accepted.

How was this change tested?

• Manually
• Unit tests
• Integration tests

Run make spelling.

Checklist

• I have added or updated any relevant documentation.
• I have cleaned any remaining cloud resources from my accounts.