Docs rework single issue notifications

What does this MR do?

This MR tried to create a single source of truth to prodive information about what "Notifications" toggle do on Issues/MRs/Epics. I think that it is important to do that because the Notifications toggle in question is located at the right sidebar which is supposed to contain mostly issue/mr/epic propperties. The Notifications toggle on the other hand changes user's settings. This can create misunderstandings of what the toggle really does for new GitLab users. That is why I think that it is important to make sure we have it documented and linked from other notification-related pages in the docs.

The MR also removes the "Ultimate" badge new the "Subscribe to and issue" feature. The Notifications toggle is available on every known plans that we have, including starter.

Related issues

Author's checklist (required)

• Follow the Documentation Guidelines and Style Guide.
• If you have Developer permissions or higher:
  • Ensure that the product tier badge is added to doc's h1.
  • Apply the documentation label, plus:
    • The corresponding DevOps stage and group labels, if applicable.
    • development guidelines when changing docs under doc/development/*, CONTRIBUTING.md, or README.md.
    • development guidelines and Documentation guidelines when changing docs under development/documentation/*.
    • development guidelines and Description templates (.gitlab/*) when creating/updating issue and MR description templates.
  • Assign the designated Technical Writer.

Do not add the feature, frontend, backend, ~"bug", or database labels if you are only updating documentation. These labels will cause the MR to be added to code verification QA issues.

When applicable:

• Update the permissions table.
• Link docs to and from the higher-level index page, plus other related docs where helpful.
• Add the product tier badge accordingly.
• Add GitLab's version history note(s).
• Add/update the feature flag section.

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• Clear typos, like this is a typpo.
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