Skip pipeline jobs if only changing docs

[sjberman]

When changing documentation in the NGF repo, we should not need to run all pipeline jobs. Only docs-related jobs should be run. This will speed up the process in changing docs and prevent unrelated failures from blocking these changes.

Even though test jobs are marked as required in Github, skipping these jobs in the workflow will still allow the PR to be merged.

We can use NIC as an example on how to skip jobs if only the docs are updated:
https://github.com/nginxinc/kubernetes-ingress/blob/main/.github/workflows/ci.yml#L60-L80
https://github.com/nginxinc/kubernetes-ingress/blob/main/.github/workflows/ci.yml#L37
https://github.com/nginxinc/kubernetes-ingress/blob/main/.github/workflows/ci.yml#L364

Acceptance

• Changes to ONLY site or docs directories should skip all build and test jobs in the pipeline.
• If changes to any other directory are detected, then all jobs should run.

[sindhushiv]

Now, the docs repo has been moved to a different repository. This was mainly done to avoid running the full pipeline in our repo when making documentation changes, which is no longer necessary. The only remaining docs in our repo are developer docs, which don't change very often.