accessibility help and policy

[cellio]

Adds accessibility help and (generic) accessibility policy (customize for your network), taken from the docs repo and translated to HTML.

[cellio]

I tested this locally by updating seeds:

UPDATE_POSTS=true rails db:seed

And then checking that the posts were there. After this is merged and deployed, we can replace the generic one on-site on our network with the Codidact-specific policy (also from the docs repo).