Update installation topic in the 'getting started' documentation

[ptitzler]

This PR removes an obsolete note from the installation documentation. With the 'recent' build/packaging changes there is no longer a need to rebuild JupyterLab. The repository README already includes a disclaimer that the rebuild is only required for Elyra version < 3.7 and therefore does not require the change.

What changes were proposed in this pull request?

• See above.

How was this pull request tested?

• Reviewed the output of make docs

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