Add linting for language structure/style

[GuySartorelli]

Initial linting was added in silverstripe/developer-docs#11 but excluded linting language style/structure.

There's a lot of tools, such as https://vale.sh/, which can help us lint our language structure.

Linting should ideally include:

• catch spelling mistakes
• catch common american-spellings that should be using the UK english
• as much of our writing style guidelines as possible
• anything else we think would be appropriate

NOTE

• We need to decide if this should apply to pre-existing changelogs (or changelogs at all) or not.

PRs

• MNT Add linting of language stucture/style developer-docs#391