Add architecture and contributor documentation

[gregpawin]

Description

Adds eight topic documents under docs/, linked from a new "Detailed documentation" section in docs/README.md.

The repository spans a Next.js app, an Express API, two Python data pipelines, and three databases, and there was no
single place explaining how those relate or which parts are actually wired together. New contributors had to infer it
from source.

Document	Covers
01-overview.md	Mission and high-level architecture
02-monorepo-structure.md	Workspace layout, tooling, local dev workflow
03-web-application.md	Next.js map app, state, Socrata integration
04-backend-api.md	Express/MongoDB API and its contract
05-data-pipeline.md	Polars + SQLite/PostGIS ingestion and schemas
06-legacy-data-science.md	Older ETL and the normalized PostGIS schema
07-data-sources-and-schemas.md	Dataset IDs, column dictionary, reference files
08-roadmap-and-open-questions.md	Known gaps and open decisions

The docs distinguish what works from what is aspirational — that the web app does not currently call the Express API, for
instance — so the diagrams are not misread as the intended end state. Anywhere behavior was unclear from source, it is
recorded as an open question in 08 rather than guessed at.

The change to docs/README.md is purely additive: it appends a section and touches nothing upstream already had.

Related Issues

Refs #695

This PR has no dedicated issue and does not implement any of #695's acceptance criteria — it documents the pipeline
work referenced there, which is why it is linked at all. Since the PR template requires an approved issue, tell me
whether you would like me to open a documentation issue for this, or whether you would rather fold these docs into the
wiki instead of the repo. Happy to close this if documentation lives elsewhere by convention.

Testing

Not applicable — documentation only, no executable code.

Verified every relative link resolves. Three of them (05-data-pipeline.md and docs/README.md pointing into
data-science/beta_pipeline/) depend on the pipeline PR, so this should merge after that one or those links will
404. Confirmed the mermaid diagrams render on GitHub.

Checklist

• I have followed all conventions outlined in our documentation.
• I have fully tested my changes and confirmed that all new and existing tests pass.
• I have written meaningful commit messages for all changes.
• I have linted and formatted my changes to follow the code style of this repository.
• I have updated our documentation, accordingly.
• I have checked currently opened pull requests to ensure that there are no pending pull request for the same
  changes or issues.
• I have confirmed that this pull request fully meets the acceptance criteria for all related issues listed above.

Last box unchecked: this is documentation and does not satisfy #695's criteria, which are about ingestion behavior.

[github-actions]

@gregpawin, this Pull Request is not linked to a valid issue. Please provide a valid linked issue in "Related Issues" above, using the format of "Resolves #" + issue number.