Add migration script for index.md links

What does this MR do and why?

Adds a script to update markdown links that reference index.md pages.

For Hugo, they need to point to _index.md instead. We already rename index files on build to _index.md (from here); this MR just fixes links that point to them.

Relates to #62 (closed)

How to set up and validate locally

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/technical-writing-group/gitlab-docs-hugo/-/blob/main/doc/setup.md.
2. Checkout this branch
3. Fetch up-to-date content, but don't run any migrations: REMOVE_BEFORE_CLONE=true go run cmd/gldocs/main.go clone
4. Run this migration only: go run cmd/gldocs/main.go migrate indexLinks ../gitlab/doc
5. View the diff: git -C ../gitlab diff

Expected result: Relative markdown links to index.md files now point to _index.md, like this:

Before:

this is a sentence with an [index page](../index.md) link

After:

this is a sentence with an [index page](../_index.md) link

Merge request acceptance checklist

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