contribute.docs.astro.build

This PR updates the process for adding (and adding to) third-party guides: CMS, backend, deploy, media. Now, the one set of instructions shown works for all of our existing third-party guide types. This updates how to add a new guide in accordance with the new schema for all third-party guides (requiring a property instead of a and other changes when all the navigation grids were updated and standardized). It also fixes the issue that the previous guidance did not actually work for adding new deploy guides (because their navigation grid component relied on updating an extra file that was not mentioned here). This also more strongly emphasizes: the need to create a basic page first with only external links and no guide content using good semantic link text when adding external links to these pages the purpose and focus of guide content, including a specific call out to making use of the section a new stub page must be created in English