Disable indexing headings in dev

[malcolm-kee]

Indexing the heading causing dev server recompilation slow.

Disable for dev by default, but expose config options if someone really want to always have it.

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🦋 Changeset detected

Latest commit: db657d4efaf437244ad2a58df44e7a023882c7e5

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 7 packages

| Name | Type | | ------------------- | ----- | | @showroomjs/core | Minor | | react-showroom | Minor | | @showroomjs/ui | Patch | | client-only-example | Patch | | react-example | Patch | | subpath-example | Patch | | react-showroom-docs | Patch |

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