Remove outdated references to alternative storage
Changes Overview: Refined documentation to remove outdated references to alternative storage methods, clarifying Hashed Storage as the sole practice in GitLab. Added a historical outline of Git repository indexing.
Comment: As Hashed Storage has become the only way to manage GitLab repositories, it might be beneficial to start purging references in ancillary locations about Hashed Storage requirements, like in Design Management. Currently, encountering "hashed storage" references might lead to assumptions about other methods. Additionally, having documentation that outlines the history of Git repositories indexing could provide valuable context.
I identified places in the documentation where references to the old storage method persisted and updated them to reflect the current standard.
Related to #430215