Adds details regarding the PDM and Releases

This bolsters guidance for Release Managers for what migrations are run when we execute the PDM. We also help describe the priority of how we treat these migrations on an everday basis. The onus resides with the development teams. Thus, so long as we are not suffering an incident where a migration is root cause, we should be confident that we are safe to perform a release. This bolsters some questions that were recently raised when very long running migrations were being run very close to a recent release.

This moves the prior generic PDM documentation to it's own page which is more runbook worthy and creates a new general page that covers most topics as a first read when landing here. Doing so also caused me to reformat the page just a teeny bit to extract the information in a concise manner.

In a prior commit, this is also fixing a bunch of random typos while building this documentation.

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Addresses: gitlab-com/gl-infra/delivery#20091 (closed)