docs: add architecture deep dive

[DhruvGarg111]

Summary

• Add a long-form Architecture Deep Dive doc with internals, diagrams, and guides
• Link it from the Architecture overview page
• Add it to the docs sidebar navigation

Test Plan

• Not run (docs-only)

Closes #673

[DhruvGarg111]

@AmitoVrito , looks like the test fail is from your side. Do check

[AmitoVrito]

@DhruvGarg111 ,that's the welcome workflow failing, not a docs build or test failure your changes are fine. It's a permissions issue in the CI config on our side, nothing to do with your PR. I will look into it and merge separately

[AmitoVrito]

Good write-up, Dhruv. The reading map at the top is a nice touch — makes it easy to jump to the section you actually need rather than reading the whole thing. The CI failure was a bug in the welcome workflow on our end, not your changes. Fixed and pushed to main. Merging now.