Text content API doesn't properly support monorepos, errors out during build if no tab name can be determined

[wise-king-sullyman]

Followup to #183 to address bugs found after pulling into org

• The apiIndex file should be placed into the outpurDir specified by the consumer rather than in the astro dir to support monorepos properly
• If no tab name can be determined (such as for local files in the consuming repo) we need to supply a fallback, as currently this situation causes the build to crash during the prerendering step. Also we should probably add some problem handling around this in case other tabless edge cases are found.

[github-actions]

🎉 This issue has been resolved in version 1.15.5 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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