feat: spellchecker CI

[vinckr]

• adds a spellcheck for US spelling to the CI, checks only .md/.mdx files in /docs.
• it does not fail when there is mistakes, but we should change that once the current misspellings are fixed.

Related Issue or Design Document

Checklist

• I have read the contributing guidelines
  and signed the CLA.
• I have referenced an issue containing the design document if my change
  introduces a new feature.
• I have read the security policy.
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security vulnerability, I
  confirm that I got green light (please contact
  security@ory.sh) from the maintainers to push
  the changes.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added necessary documentation within the code base (if
  appropriate).

Further comments

[tomekpapiernik]

LGTM! It's gonna be a nice addition to this repo :)