Also updates redirect text in /project-services

For the sake of consistency, removes any extraneous
consecutive blank lines from the doc suite.

Clarify the language around External Group membership with SAML SSO to clarify that this will NOT add users to GitLab Groups.

This commit lets a user bypass the automatic signin on the login form,
in order to login with a technical (admin, etc) account

Closes #3786
Signed-off-by: NRémy Coutable <remy@rymai.me>

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