Add ADR 1 for SD blueprint, outline future approaches

What does this MR do and why?

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Architectural Decision Record

Per this handbook page I'd like to capture our decision around the initial iteration of pre-receive Category:Secret Detection within the blueprint.

While the high-level content remains correct, the Architectural Decision Record (ADR) helps provide context towards the decision for the future. Most of this is distilled from the Pre-receive Secret Detection – Decision & FAQs document, but my aim was for a lighter summary with an emphasis on the two main concerns: implementation language and distribution mechanics. This is certainly a draft as the original document authors @ahmed.hemdan and @rossfuhrman deserve the credit for documented work and perhaps it would be best to capture the contents 1-1 instead, love to get their perspective.

Design Stages

These outline what the post-experimental phase could look like so the next stages have a clear starting point. Again, these need better numbers but the pre-production profiling should update these with target sizes, headroom, ref's architectures, etc.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.