Improve GitLab.com SAML documentation: links, bullet points and clarify .com

What

• Makes it clearer that the Group SAML docs refer primarily to GitLab.com
• Makes NameID requirements easier to scan by using bullet points
• Adds links between instance/group SAML so users can more easily find the correct docs

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/58637, which is the docs review this evolved from

Does this MR meet the acceptance criteria?

• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created