Update Publishing & Federation list with ATmosphere
Keep the implementations index current as new standard.site
implementations ship.
Two changes to content/docs/implementations.mdx:
1. Rename "Migration Tools" to "Publishing & Federation". All three
entries in the section (sequoia.pub, wireservice, ATmosphere) are
continuous publishing/federation tools that write standard.site
records to a PDS on an ongoing basis — none of them is a one-time
migration utility, so the old label was misleading.
2. Add ATmosphere (https://wordpress.org/plugins/atmosphere/) above
wireservice. The card carries an explicit title/description so the
distinguishing trait surfaces without depending on OG metadata:
ATmosphere talks directly to the PDS via OAuth 2.1 with PKCE and
DPoP, with no third-party proxy in the auth path — wireservice by
contrast routes OAuth through an external service (aip.wireservice.net
by default).