Fix the Unstable Docs Version Warning banner to Header

[iam-veeramalla]

Checklist:

• I've searched in the docs and FAQ for my answer: https://bit.ly/argocd-faq.
• I've included steps to reproduce the bug.
• I've pasted the output of argocd version.

Describe the bug
The Unstable docs version warning banner is not fixed to the header.

When someone lands on Old/Unstable docs page from google search they see a warning banner and a link to stable docs, which is good. However this disappears as they scroll down on the docs, there is a good chance that someone would not see it on a very first sight and scroll down.

Also, I would suggest if we can change the warning/recommendation banner to a strong font and colour.

To Reproduce

Google search for the Argo CD Docs would mostly land you on the older version of docs and you should see the issue that I mentioned in the description.

Screenshots

Version
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Logs
NA

Paste any relevant application logs here.

[iam-veeramalla]

@reginapizza, @saumeya I think this should be an easy bug to fix If I am not mistaken :)

[saumeya]

I'll take a look.

[saumeya]

#6682
@reginapizza Please could you review.