catalog,catalog-react,techdocs: extract experimental entity page implementation from app-next

Co-authored-by: Camila Belo <camilaibs@gmail.com>
Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

.changeset/mighty-crews-attack.md

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+---
+'@backstage/plugin-catalog': patch
+---
+
+Initial entity page implementation for new frontend system at `/alpha`, with an overview page enabled by default and the about card available as an optional card.

.changeset/nice-apes-kneel.md

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+---
+'@backstage/plugin-techdocs': patch
+---
+
+Added entity page content for the new plugin exported via `/alpha`.

.changeset/old-apricots-taste.md

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+---
+'@backstage/plugin-catalog-react': patch
+---
+
+Added new APIs at the `/alpha` subpath for creating entity page cards and content for the new frontend system.