Move `schema.json` from `validation` to `docs`

[JamieMagee]

This allows the JSON schema to be published alongside the rest of the documentation to GitHub pages. The schema will be available at https://ossf.github.io/osv-schema/schema.json.

Also created a symlink to the previous repository location of validation/schema.json in case anyone is already using that in a $schema property

Closes #226