pgamdirect: add redirect_from, remove legacy pgam + pgammedia pages (#6565)

Follow-up cleanup to #6536 (which landed pgamdirect.md with GVL 1353).
Two improvements that didn't make the original merge:

1. Add `redirect_from` to pgamdirect.md so historical links to
   `/dev-docs/bidders/pgam` and `/dev-docs/bidders/pgammedia` resolve
   to the new canonical page instead of 404-ing once the legacy
   pages are removed.

2. Remove the legacy pgam.md and pgammedia.md pages. Both are stale:
   - pgam.md: biddercode `pgam`, marked `removed in 8.13.0` — has
     not had a working adapter in prebid-js master for releases.
   - pgammedia.md: documents an aniview alias (`aliasCode: aniview`)
     for an adapter PGAM Media no longer operates. The live PGAM
     bidders are pgamssp (TeqBlaze-hosted, still maintained for
     legacy partners) and pgamdirect (this page, self-hosted).

@muuki88 cherry-picked the redirect commit on a prior branch
(ba009a0) — re-submitting it here as a clean two-commit PR against
the current master post-#6536 merge.

Lint: markdownlint-cli passes against the repo's `.markdownlint.json`
with 2-space (not tab) indentation in the redirect_from block.

Author: mastap150

dev-docs/bidders/pgam.md

@@ -2,6 +2,9 @@
 layout: bidder
 title: PGAM Direct
 description: Server-to-server OpenRTB 2.6 adapter from PGAM Media (self-hosted). Dynamic floors, transparent margins, schain integrity. Related — pgamssp (legacy, TeqBlaze-hosted).
+redirect_from:
+  - /dev-docs/bidders/pgam
+  - /dev-docs/bidders/pgammedia
 biddercode: pgamdirect
 media_types: banner, video, native
 gvl_id: 1353