docs(members): point member doc links at the group and project members pages

What does this MR do?

GitLab split the members API docs (/api/members/) into /api/group_members/ and /api/project_members/, and the old section titles changed. All 33 doc links in group_members.go and project_members.go still point at https://docs.gitlab.com/api/members/#...:

• /api/members/ now redirects to /api/group_members/, so every ProjectMembersService link opens the group page.
• None of the old anchors (#get-a-member-of-a-group-or-project, #edit-a-member-of-a-group-or-project, and so on) exist on either new page, so the links land at the top of the page.

This MR maps each link to its new section, matching on the endpoint each function calls:

Function(s)	New section
ListGroupMembers	group_members/#list-all-group-members
ListAllGroupMembers	group_members/#list-all-group-members-including-inherited-and-invited-members
GetGroupMember / GetInheritedGroupMember	group_members/#retrieve-a-group-member / ...-including-inherited-and-invited-members
AddGroupMember, EditGroupMember, RemoveGroupMember	#add-a-group-member, #update-a-group-member, #remove-a-group-member
billable member functions	#list-all-billable-group-members, #list-all-memberships-for-a-billable-group-member, #remove-a-billable-group-member
ListProjectMembers / ListAllProjectMembers	project_members/#list-all-direct-members-of-a-project / #list-all-members-of-a-project
GetProjectMember / GetInheritedProjectMember	#retrieve-a-direct-member-of-a-project / #retrieve-a-member-of-a-project
AddProjectMember, EditProjectMember, DeleteProjectMember	#add-a-member-to-a-project, #update-a-member-of-a-project, #remove-a-direct-member-of-a-project

How was this checked?

• Every new anchor (17 distinct) was checked against the heading ids on the live docs pages, and against doc/api/group_members.md / doc/api/project_members.md in gitlab-org/gitlab.
• Each mapping was checked against the route the function uses (for example ListAllProjectMembers -> projects/%s/members/all -> "List all members of a project").
• Comment-only change.

This contribution was prepared with the help of an AI assistant. I reviewed the changes and take responsibility for them.