website: update main branch archive banner

[thesuperzapper]

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Description of your changes:

This PR updates the archive banner text so when archived_version=true it's more clear that the branch is archived.

NOTE: this PR only updates the templates so that when we next fork off the main branch to a v1.11-branch it is easy to just set archived_version=true

NOTE 2: this is not a new change, it has already been done to all the old branches, but this means its less work for future versions.

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/area website

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[thesuperzapper]

@varodrig this is important to update the archive banner on the main branch (but not set it because the main branch is obviously not archived!).

It also adds information about what to change when cutting a release branch (so that the version of the branch shows at the top right of the page).