Implement Vale content linting for Release Posts

Problem to solve

Currently PMs and other participants in the release post process spend a significant amount of time trying to adhere to correct general spelling/grammar and Gitlab specific stylistic standards for the content. It's not a successful effort given the volume of content, so typos, URLs and other other stylistic missed still slip through. And in some cases, due to amount of content that needs review, not all the content can be copy edited or revised for stylistic consistency.

Proposal

Define a list of rules to be validated by Vale, an open source linter for content. Run this validation as part of the CI pipeline and surface any warnings or errors appropriately in the Merge Request page.