docs: clean up and complete the JSIR documentation

[rootkiller6788]

Summary

Completes the JSIR documentation cleanup tracked in #9. The docs previously
contained a stale dialect reference, a missing dataflow-analysis API reference,
unresolved work-in-progress phrasing, and leftover [TOC] placeholders. This
change fixes those and documents the jsir / jshir dialect split.

Changes

• README.md

  • Fix a stale JSHIR reference: the jsir_gen tool produces the full IR
    (both dialects), not the high-level dialect alone, so the heading now says
    JSIR.
  • Link docs/dataflow_analysis.md alongside the existing IR design doc.

• docs/intermediate_representation_design.md

  • Add a "Two dialects: jsir and jshir" section explaining that jshir is
    the high-level IR (HIR, control-flow ops) and jsir is the low-level IR
    (LIR, non-control-flow ops), per maldoca/astgen/ast_def.h.
  • Replace the in-progress symbol-representation note with a link to Revisit the representation of lvalues, especially patterns #25.

• docs/dataflow_analysis.md

  • Add a "The C++ API" section covering JsirState<T>, JsirPerVarState,
    the analysis base classes, and the RunJsirAnalysis(...) entry point.
  • Add the lattice-order definition for constant propagation
    (Uninit ≤ Const{N} ≤ Unknown).
  • Replace work-in-progress phrasing with links to tracking issues.
  • Remove the leftover [TOC] placeholder (also removed from
    intermediate_representation_design.md).
  • Clarify the jsir / jshir op naming and Uninit (uninitialized)
    terminology.

References

• Closes Clean up and release documentation #9
• See Revisit the representation of lvalues, especially patterns #25 for the ongoing lvalue representation review

Notes

• Doc-only change; no code or build changes.
• Signed the Google CLA (required before this can be merged).

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[phisiart]

Thanks for the contribution! Please note that your PR might be delayed by several days because (1) we still haven't established a way to automatically sync GitHub PRs into our internal codebase, so we will have to manually do it; (2) our internal codebase -> GitHub sync process is somewhat unstable, so we might need to fix that first.

[phisiart]

jsir does not mean low-level. Maybe change the two bullet points into:

• jshir holds the operations that carry control-flow (branch) semantics...
• jsir holds the operations without control-flow semantics...