docs: added Node.js 12 deprecation banner to docs

[dreamorosi]

Description of your changes

This PR introduces a banner at the top of the documentation website to inform customers that v1.5.0 will be the last version that supports Nodejs 12 and the nodejs12.x AWS Lambda runtime. While we don't have a date scheduled for the next release, I don't expect there to be any release before Jan 2023 (over 2weeks away).

In addition to the banner, I have left a longer comment in the linked issue and made an announcement on the Discord server, as well as Twitter.

How to verify this change

See docs local preview or published docs after merge.

Related issues, RFCs

Issue number: #1061

PR status

Is this ready for review?: YES
Is it a breaking change?: NO

Checklist

• My changes meet the tenets criteria
• I have performed a self-review of my own code
• I have commented my code where necessary, particularly in areas that should be flagged with a TODO, or hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding changes to the examples
• My changes generate no new warnings
• The code coverage hasn't decreased
• I have added tests that prove my change is effective and works
• New and existing unit tests pass locally and in Github Actions
• Any dependent changes have been merged and published
• The PR title follows the conventional commit semantics

Breaking change checklist

• I have documented the migration process
• I have added, implemented necessary warnings (if it can live side by side)

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