Fix invalid URL in extension descriptions

[ptitzler]

The following Elyra stand-alone extension pages on PyPI contain a documentation link, which is an invalid URL:

• https://pypi.org/project/elyra-python-editor-extension/
• https://pypi.org/project/elyra-r-editor-extension/
• https://pypi.org/project/elyra-code-snippet-extension/
• https://pypi.org/project/elyra-pipeline-editor-extension/

The issue is caused by the release script https://github.com/elyra-ai/elyra/blob/main/create-release.py#L579-L611, which uses https://elyra.readthedocs.io/en/{config.new_version}/ as base URL instead of https://elyra.readthedocs.io/en/v{config.new_version}/

What changes were proposed in this pull request?

See above.

How was this pull request tested?

Manually confirmed that the new URL is valid.

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