[DOCS] Separate "user lookup" into its own doc

[tvernum]

When we introduced "authorization delegation" we piggy backed on the implementation and documentation for "run as". The authorization delegation docs just link to "run as" in order to explain which realms support being the target of delegation.

However, authorization delegation is now at least as popular as "run as", and forcing people to make sense of the "run as" docs and apply them to their delegation authorization ("authorization_realm") scenario wasn't helpful or clear.

This commit moves (and improves) the content for "lookup a user without authentication" into a new page within the authentication section, and links to it from both "run as" and "delegated authorization".

Preview link: https://elasticsearch_88533.docs-preview.app.elstc.co/guide/en/elasticsearch/reference/master/user-lookup.html

[elasticmachine]

Pinging @elastic/es-docs (Team:Docs)

[elasticmachine]

Pinging @elastic/es-security (Team:Security)

[justincr-elastic]

LGTM

[tvernum]

@elasticmachine update branch

[tvernum]

@elasticmachine update branch

[elasticsearchmachine]

💚 Backport successful

Status	Branch	Result
✅	8.3
✅	8.4