Associate user doc to custom component

[yuhuishi-convect]

What changes were proposed in this pull request?

Associate user comment nodes to custom components (kfp and airflow).
Fixes #2458

How was this pull request tested?

make test-server

Tested from UI as well

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[elyra-bot]

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[kiersten-stokes]

LGTM - thank you for another awesome contribution, @yuhuishi-convect!

[ptitzler]

LGTM! Thank you!