docs: configure spell check

[sarahschwartz]

This PR configures the spell-check added to the docs workflow in the github-actions repo: FuelLabs/github-actions#23

The files checked are configured in .spellcheck.yml. This is also where you can configure what types of elements are ignored.

Right now it ignores:

• All code blocks that have a language (and will check code blocks that do not have a language)
• Anything in between backticks
• Words in spell-check-custom-words.txt (case sensitive, only exact match)
• Numbers, even if they are attached to a word
• Links in markdown link format

Handling errors:

If the test fails:

• look up the word in the question to verify it is a real word and is correctly spelled
• If it is a file name or is code, use backticks to ignore the word.
• If it is a real word that is spelled correctly, or an acronym that is either common or is defined already, add it to spell-check-custom-words.txt.
• If needed, rewrite the sentence. Ex: DON'T use "locking" and add "ing" to the custom words list. Instead, rewrite the sentence as "locking with the lock method".
• If it otherwise should be ignored, you can configure the pipeline in .spellcheck.yml.

[Dentosal]

Some minor nits. You can fix those if it feels worth doing, but merging as-is is fine for me.