Update RTD config to remove empty build.tools

[sirosen]

...and enable the RTD config linting hook.

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This was noted during #2273, so I reached out to RTD for clarification.
Based on this response, it seems safe to assume that empty and missing are the same for build.tools (that's the logical thing to guess as well).

Regardless of whether or not the schema gets updated -- it probably won't -- we can enable the hook today if we remove build.tools.

Contributor checklist

• [x] Included tests for the changes.
• A change note is created in changelog.d/ (see changelog.d/README.md for instructions) or the PR text says "no changelog needed".

Maintainer checklist

• If no changelog is needed, apply the bot:chronographer:skip label.
• Assign the PR to an existing or new milestone for the target version (following Semantic Versioning).

[webknjaz]

brilliant!