Document differences between spelling tests

We have two spelling tests in use:

• Markdownlint, in .markdownlint.yml in the root directory of the gitlab repo. Breaks CI on failure.
• Vale, in doc/.vale/gitlab/Spelling.yml (the rule) and doc/.vale/gitlab/spelling-exceptions.txt (allowed exceptions). Does not break CI.

@msedlakjakubowski reminded me in gitlab!92082 (comment 1020882495) that we have some tech debt here. The Markdownlint spelling test predates Vale, and probably contains items that should be downgraded from CI-breaking to a warning.

We should:

1. Scan through both word lists.
2. Identify which ones we want to break CI on. I expect we'd prioritize our company name, common product names, and multi-word phrases with specific capitalization needs. Keep those in the Markdownlint test.
3. Move everything else into the Vale spelling-exceptions list. Correct any new problems that block pipelines.
4. Document the setup in our style guide and clearly explain the difference between the two lists.

Related issues

• Related to gitlab!92082 where Marcin prompted me.