Add API documentation for guest api new endpoints

Description

Document the new endpoints added in the PRs:

  • pr-mattermost-server-10792

  • pr-mattermost-server-10791

QA Test Steps

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Status

Mana

None

Assignee

Jesús Espino

QA Assignee

None

Reporter

Jesús Espino

Epic Link

None

Fix versions

Mattermost Team

Platform

Labels

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QA Testing Areas

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GitHub Issue

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