Add design documents for Active Context and Code Embeddings

Why is this change being made?

There has been a lot of updates to ActiveContext, the framework on which Semantic Code Search is implemented. This MR reflects those changes:

• added decision record indicating that the "AI Context Abstraction Layer" is implemented in a gem called ActiveContext
• added decision record for the ActiveContext Tasks framework + design document with further details
• added decision record for the Active Context Embedding Model redesign + design document for further details

Additional change

• design document for the Chunking component of the Code Embeddings Indexing pipeline

Author and Reviewer Checklist

Please verify the check list and ensure to tick them off before the MR is merged.

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
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• Assign reviewers for this MR to the correct
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Commits

• Add design documents for Semantic Search

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