Add symlink for schema.json to docs (#232)

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

Signed-off-by: Jamie Magee <[email protected]>

Author: JamieMagee

docs/schema.json

@@ -0,0 +1 @@
+../validation/schema.json