Add find-unused-images task

What does this MR do and why?

This adds an updated monthly task to look for unused images. This is written in go and is 10x more efficient than the one we used to use before.

This MR also introduces a new docs page for monthly tasks, so we can add details about each task as we add them.

Related to Update documentation for monthly maintenance tasks (#270 - closed)

Screenshots, screen recordings, or links to review app

• Old job: https://gitlab.com/gitlab-org/gitlab-docs/-/jobs/7719086484 (30 minutes)
• New job: https://gitlab.com/gitlab-org/technical-writing/docs-gitlab-com/-/jobs/9397851512 (3 minutes)

How to set up and validate locally

Usage examples:

• go run cmd/gldocs/main.go find-unused-images --verbose
• go run cmd/gldocs/main.go find-unused-images --dir ../gitlab/doc

Command line flags:

--dir, -d string      Directory to search for images and markdown files
--verbose, -v         Enable verbose output
--workers, -w int     Number of worker goroutines to use
--exit-code, -e int   Exit with non-zero code when unused images are found
--help, -h            Help for find-unused-images