Improve documentation for Delayed Project Deletion

Problem to solve

Currently documentation for "delayed project deletion" feels incomplete. This is especially visible right now because of the effort to mitigate accidental project deletion, which may rely on "delayed project deletion".

Specifically, it feels like there is currently no SSOT for delayed project deletion. That seems to have led to documentation cross-referencing in ways that feel unexpected/unclear. Also, we've found some documentation is using deprecated terminology, and some documentation is missing, as a result of multiple engineers contributing to this feature set over time.

Proposal

• Add a documentation section to serve as the SSOT for the "delayed project deletion" feature set. This section should explain the feature, and all the available settings. !74721 (merged)
• Link to the SSOT in other documentation: !75684 (merged)
• Replace "remove" with "delete" for clarity and consistency: !74640 (merged)
• Add missing documentation for admin Deleted Projects page: !74652 (merged)

Overview of existing documentation

Admin level

• Setting: Default enabled for new groups
• Setting: Default delay

Group level

• Setting: Enable delayed project removal

Project level

NONE

User

None

GitLab.com

• Setting: Delay enabled by default (due to group-level default?)