docs: the documentation home opens with the command-line banner

[nicolasbisurgi]

The documentation home page now opens with the OptimusPy command-line banner, the same way the RushTI documentation opens with its own.

• The image (docs/assets/images/optimuspy/optimuspy-cli.png) sits at the top, centred and capped at 560px wide, and scales down to fit on a phone.
• Below it, the introduction is plain text: what OptimusPy does and the supported versions. The blue title banner is gone from this page, along with its "Back to the landing page" link, because the Website button just below it goes to the same place.
• The rest of the page is unchanged. The browser tab still reads "OptimusPy Documentation".

python -m mkdocs build --strict is clean, and the page was checked in a browser at desktop and phone widths.

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