Hide design docs from documentation

[agjohnson]

I constantly hit these pages and they must be very confusing for
customers and non-technical users. These are internal conversations and
should probably be moved to a separate location entirely, but for now,
this will hide them from output and hopefully from indexing. If this
does not remove them from indexing, I think they should be removed from
this doc set entirely.

Most commonly, I hit the APIv3 design doc as the first search result,
before the actual APIv3 documentation.

[agjohnson]

Search indexing of these pages can't be testing the PR build, but the design docs are still built, and do not emit an error on build. I can test search index outside a PR build.

[agjohnson]

With the search ignore addition, the design docs still build, are not linked in the /developer/ page, and will not show up in search results.

If we want to step this back, we can still build and link the files in /developer/, but ignore in search results. I'd still probably vote these are not good permanent docs to include in our documentation however, as they were mostly used to enable conversation around feature design.

[stsewd]

I'm fine actually having a link to them in the developer docs, is nice to have this rendered and easily to find, since now this isn't going to appear in the search results.

[ericholscher]

I think we should leave these in the TOC, but just exclude them from search. Otherwise they'll be impossible to find.