Update blog submission guidelines around links into repositories

follow up to https://github.com/open-telemetry/opentelemetry.io/pull/7513:

We should add a section to the blog submission guidelines that links to github repos should not point to `main` (or an equivalent default branch) but to a commit or release that is up-to-date point in time. This way we avoid links breaking in the future when we no longer update the blog posts.