Refactor file locking docs

What does this MR do?

There are two forms of file lock supported by GitLab. These were documented in two different location. This merge request centralizes all file locking documentation in a single location, and explains the difference.

Detailed documentation has been added to explain how the git-lfs file locking commands can be used not only for LFS files, but any file. This is important for partial clone workflows involving large files.

More context:

• exclusive file locking (file is read only until write permissions explicitly requested) is the model primarily used by people working on binary files in Perforce, and was added to Git LFS for this reason.
• the default-branch file locking feature was seemingly added for this reason according to #497 (closed), but didn't really deliver on this workflow. Possibly to make the first iteration smaller.
• the iteration to add Git LFS support probably should have replaced and upgraded the default-branch mode, rather than add a second mode of locking
• there is a proposal to deprecate and consolidate them in #224481 (closed)

Related issues

#219957 (closed)

Author's checklist (required)

• Follow the Documentation Guidelines and Style Guide.
• If you have developer access or higher (for example, GitLab team members or Core Team members)
  • Apply the documentation label, plus:
    • The corresponding DevOps stage and group label, 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 GitLab's version history note(s).
• Add the product tier badge.
• Add/update the feature flag section.
• If you're changing document headings, search doc/*, app/views/*, and ee/app/views/* for old headings replacing with the new ones to avoid broken anchors.

Review checklist

All reviewers can help ensure accuracy, clarity, completeness, and adherence to the Documentation Guidelines and Style Guide.

1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

2. Technical Writer

• Optional: Technical writer review. If not requested for this MR, must be scheduled post-merge. To request for this MR, assign the writer listed for the applicable DevOps stage.
  • Add Technical Writing and docs:: workflow label.
  • Add docs-only when the only files changed are under doc/*.

3. Maintainer

1. Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
2. Ensure a release milestone is set.
3. If there has not been a technical writer review, create an issue for one using the Doc Review template.