docs: Remove duplicated "Using the Pro Client" section and point towards proper documentation.

[Monochromics]

"Using the pro client" is primarily a duplication of the content at https://canonical-ubuntu-pro-client.readthedocs-hosted.com/en/latest/howtoguides/#

The linked document contains the content described in the pro docs, along with significant additional content. It would be better in my opinion to just redirect folks to the proper documentation to keep things easier to maintain.

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*Reported from: https://documentation.ubuntu.com/pro/how-to-guides/