feat: Support migration guides for enterprise connectors

[ian-at-airbyte]

What

This PR adds support for migration guides on enterprise connectors.

How

Updated filtering logic to support top-level id as well as nested link.id.

The existing filtering logic for enterprise connectors in the sidebar did not expect nested migration guides. The reason is because the getFilenamesInDir() function returns two different object structures.

Simple doc (no migration file): { type: "doc", id: "...", label: "..." } - has id at top level
Category with migration doc: { type: "category", label: "...", link: { id: "..." }, items: [...] } - has id nested in link.id

@darynaishchenko discovered during work on #67214 that all builds failed when a migration guide was added to an enterprise connector.

Review guide

1. Review the logic in the sidebar filtering code.
2. There is currently no migration guide in master to test this on, but I did test this fix with Darnya's PR and it built correctly with the updated code. You can simulate a migration guide by adding a dummy <connector_filename>-migrations.md doc for an enterprise connector. At build time it should nest appropriately under its parent object.

User Impact

Enterprise customers can have migration guides too.

Can this PR be safely reverted and rolled back?

• YES 💚
• NO ❌

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[letiescanciano]

LGTM! Thanks for fixing :)