Add GitHub Action to check spelling

[aaronraimist]

Works the same way as matrix-org/matrix-spec-proposals#3591

This uses https://github.com/crate-ci/typos so it will only find known misspellings in their dictionary. It won't know about or correct Matrix specific terminology but I think this should find most of the misspellings.

When it finds a typo it looks like this https://github.com/aaronraimist/matrix-doc/runs/4621564484?check_suite_focus=true

When there is a false positive, like a room ID, it can be added to the exclude list in https://github.com/matrix-org/matrix-doc/blob/1f41c6db6c943f19643dbeba31fb045d81da9728/.github/_typos.toml

Preview: https://pr1226--matrix-org-previews.netlify.app