Follow-up: Documentation for self-managed SCIM

We added bare bones documentation for instance SCIM at !109127 (merged)

Some suggested additions:

• add section for linking SCIM and SAML identities similar to https://docs.gitlab.com/ee/user/group/saml_sso/scim_setup.html#link-scim-and-saml-identities but specific to Instance SCIM
• add a section on removing access for SCIM users similar to https://docs.gitlab.com/ee/user/group/saml_sso/scim_setup.html#remove-access
• include instructions specific to to setting up Azure/Okta/OneLogin (most likely take content from https://docs.gitlab.com/ee/user/group/saml_sso/scim_setup.html#configure-an-identity-provider and put it in a centralized place for reference on setting up both instance and group SCIM)
• include troubleshooting SCIM section similar to https://docs.gitlab.com/ee/user/group/saml_sso/troubleshooting_scim.html (or share that content between the group and instance SCIM docs)
• Create sample screenshots for instance scim, similar to https://docs.gitlab.com/ee/user/group/saml_sso/example_saml_config.html