Update 'pipeline components' topic in the 'user guide'

[ptitzler]

This PR updates the https://elyra.readthedocs.io/en/latest/user_guide/pipeline-components.html topic based on the changes delivered by #2241

• introduce component catalogs and component catalog connectors
• link to the component catalog marketplace
• link to the BYO component catalog tutorial in the examples repository
• update the custom component management reference
• add reference for built-in component connectors

What changes were proposed in this pull request?

See above.

Some of the HREFs to the examples repository will be broken until elyra-ai/examples#78 was merged.

How was this pull request tested?

Reviewed the output of make docs

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[ptitzler]

TODO: we need to update the CLI examples in section 'Managing custom components using the Elyra CLI' to reflect the latest changes

[kevin-bates]

reminder: this will need to be updated once the examples are moved out of the repo.

[kevin-bates]

We might as well circle back on this once the dust has settled on the various PRs.

[ptitzler]

The documentation updates will be delivered in #2286