Align bot user for projects/groups term within docs

What does this MR do?

Follow-up to !77449 (comment 809016666)

This MR aligns the bot user for projects/groups terms for all project/group access token service accounts within the documentation. This should clarify the different types of service account as well as making the correlation between bot users for projects and groups better understandable.

Besides that, this MR also:

• Adds a missing chapter about group access tokens to the token overview page, describing features and limitations (see doc/security/token_overview.md).
• Corrects the linking within the group access token REST API page to use group instead of project access token scopes (see doc/api/group_access_tokens.md).

/cc @eread

Related issues

• !77449 (merged)

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