Adding some missing objects to inheritance flowchart

What does this MR do and why?

Added missing content to inheritance flowchart

Before:

flowchart TD
    Group -->|Contains| Project
    Group -->|Contains| Epics
    Group -->|Contains| Labels
    Group -->|Contains| Boards
    Group -->|Contains| Iterations
    Project -->|Contains| Issues
    Labels .->|Cascades To| Project
    Issues .->|Rolls Up To| Group
    Iterations .->|Cascades To| Project
    Issues .->|Assigned To| Epics
    Issues .->|Visible In| Boards
    Issues .->|Assigned To| Iterations

After:

%%{init: { "fontFamily": "GitLab Sans" }}%%
flowchart TD
    accTitle: GitLab inheritance model diagram
    accDescr: Shows how groups, projects, issues, labels, milestones, iterations, tasks, and epics relate to one another in GitLab
    Group -->|Contains| Project
    Group -->|Contains| Epics
    Group -->|Contains| Labels
    Group -->|Contains| Boards
    Group -->|Contains| Iterations
    Group -->|Contains| Milestones
    Group -->|Contains| Roadmaps
    Project -->|Contains| Issues
    Project -->|Contains| Templates
    Project -->|Contains| Tasks
    Project -->|Contains| Milestones
    Project -->|Contains| Labels
    Labels .->|Cascades To| Project
    Issues .->|Rolls up to| Group
    Iterations .->|Cascades to| Project
    Milestones .->|Cascades to| Project
    Templates .->|Cascades to| Project
    Templates .->|Configured in| Group
    Issues .->|Child of| Epics
    Issues .->|Visible in| Boards
    Issues .->|Visible in| Lists
    Issues .->|Assigned to| Iterations
    Issues .->|Assigned to| Milestones
    Tasks  .->|Child of| Issues
    Tasks .->|Assigned to| Iterations
    Tasks .->|Assigned to| Milestones
    Epics .->|Visible in| Boards
    Epics .->|Visible in| Roadmaps
    Epics .->|Visible in| Lists

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Before	After

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