Docs: New Bidder — PGAM Direct (#6536)

* Docs: New Bidder — PGAM Direct

Adds the docs page for the new pgamdirect bidder adapter.

Code PR: https://github.com/prebid/Prebid.js/pull/14763

GVL 1353. Related to existing pgamssp adapter; this doc explicitly
calls out the relationship so publishers browsing the bidders list
understand which adapter is which.

* Update dev-docs/bidders/pgamdirect.md

* Update dev-docs/bidders/pgamdirect.md

* Update dev-docs/bidders/pgamdirect.md

* Update dev-docs/bidders/pgamdirect.md

* Update dev-docs/bidders/pgamdirect.md

* Update dev-docs/bidders/pgamdirect.md

* Update dev-docs/bidders/pgamdirect.md

---------

Co-authored-by: Muki Seiler <muuki88@users.noreply.github.com>