Make it possible to hide macros in docs

[alf-mindshift]

resolves #2430

Checklist

• I have signed the CLA
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• I have updated the CHANGELOG.md and added information about my change to the "dbt next" section.

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[beckjake]

@cla-bot check

[cla-bot]

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[beckjake]

@alf-mindshift, thanks for your contribution! The unit tests around serialization/deserialization are going to fail, and you'll probably also have to fix the docs generate tests, but this idea generally looks good! Please don't forget to add yourself as a contributor and this PR to CHANGELOG.md when this is ready, so we can give you credit in the release notes.

[alf-mindshift]

I rebased on top of octavius-catto. Let me know if I should rebase on top of marian-anderson instead.

[beckjake]

I rebased on top of octavius-catto. Let me know if I should rebase on top of marian-anderson instead.

Please do! octavius-catto is really close to release, so it's bugfixes only right now. Once you do that I'll kick off the tests again

[beckjake]

Assuming the Windows tests pass, as I assume they will, I'm going to repair that merge conflict and merge. There's no need to go through all this again for a changelog conflict!