docs: add banner and toggle config for previous versions

[zr-msft]

• Updated style and HTML to always have previous versions banner available.
• Added toml config to disable by default. This can be changed to enable when newer release is available.
• Updated readme with correct instructions

Signed-off-by: Zach Rhoads zachary.rhoads@microsoft.com

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[zr-msft]

Feedback from @keithmattix reminded me to remove old release versions from the versions list in main. This avoids carrying unsupported versions when we cut a release branch