Deprecate XAAS page on Vuepress

[annejuan-okta]

Description:

• What's changed? Deprecate identity sources on VuePress, added a missing Postman paragraph to Schemas page
• Is this PR related to a Monolith release? Nope

Do not merge until the Redocly version of Identity Sources is published.

Resolves:

• OKTA-734762

[acrolinxdevdocs-okta]

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Article	Total score Required:80	Word and phrases (Brand, terms) Preferred: 80	Correctness (Spelling, grammar) Preferred: 80	Clarity (Readability) Preferred: 80	Inclusive language (+ accesibility) Preferred: 80
:white_check_mark: packages/@okta/vuepress-site/docs/guides/anything-as-a-source/index.md	81	100	83	72	100

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