Document submodules handling

What does this MR do?

Documents handling of submodules and FF_GIT_URLS_WITHOUT_TOKENS

Why was this MR needed?

What's the best way to test this MR?

What are the relevant issue numbers?

Closes Document how submodules can be updated as part ... (#38815) • Georgi N. Georgiev | GitLab

Closes Kubernetes executor cannot clone git submodule (#36856 - closed) • Georgi N. Georgiev | GitLab • 18.2