Add docs and example for decoder construction from Stim DEM strings

[vedika-saravanan]

Description

Updates QEC docs and adds public C++ and Python examples for decoder construction from Stim DEM strings.

Closes: #595

Runtime / performance impact

N/A. Documentation/example-only change.

Self-review checklist

Please confirm each item before requesting review. Check [x] or strike
through and explain.

Before requesting review

• I reviewed my own full diff in GitHub or my editor.
• PR is in Draft if it is not yet ready for review.
• Temporary / debugging changes have been removed.
• Local test logs reviewed; no unexplained warnings or errors.
• CI logs reviewed; no unexplained warnings or errors.
• Full CI has been run.

Scope and size

• PR is under ~1000 lines, or an exception is justified in the description.
• Refactoring-only changes are isolated in their own PR(s).
• No existing tests were disabled or modified just to make this PR pass
  (if so, an issue has been raised).

Tests

• New functionality has new tests: Docs/example-only PR; runtime functionality is covered by Support Stim DEM strings in get_decoder #571 tests.
• Tests fail if the new functionality is broken (including crashes), not
  just when it is missing: N/A, no runtime behavior added.
• Negative tests added where exceptions are expected: N/A
• Truth data added where simple EXPECT_* / assert checks are
  insufficient for algorithmic correctness: N/A
• CI runtime impact considered; team notified if significant.

Documentation

• Public-facing APIs have Doxygen docs.
• User-visible behavior changes have public docs, or a follow-up is
  tracked.

Code style

• Naming follows the existing convention (snake_case vs camelCase) for
  the area being modified.

Dependencies

• No new third-party dependencies, or the team has been notified and
  OSRB tickets filed.