Docs: dead source links in the state migration guide and the tracer pages

Two broken source links in the docs, both verified against the contents API rather than a local checkout. Your AI policy says PRs may only target accepted issues, so I am asking first — happy to send the PR if you accept this.

**1. `docs/docs/developers/guides/state_migration_guide.md:195`** links

`github.com/ChainSafe/forest/blob/main/blockchain/state_manager/src/lib.rs`

which 404s; that path predates the move to `src/`. The state manager is now `src/state_manager/mod.rs`.

The sentence is stale beyond the URL, though:

> This is done in the `handle_state_migrations` method in the [state manager](…).

`handle_state_migrations` does not appear anywhere in the tree — a code search finds it only in this doc. The current entry point looks like `run_state_migrations`, defined in `src/state_migration/mod.rs` and invoked from `src/state_manager/state_computation.rs`. I would rather you confirm which of those the sentence should name than guess, since it is describing where a migration gets hooked in at the right height.

**2. Five links to `Tracer.sol` pinned at a commit that is not in the repository.** All five use

`blob/963237708137e9c7388c57eba39a2f8bf12ace74/src/tool/subcommands/api_cmd/contracts/tracer/Tracer.sol`

and the API answers `No commit found for SHA: 963237708137e9c7388c57e…`. The file itself is fine on `main` (11,138 bytes), so the fix is just the ref. They are in:

- `docs/docs/users/knowledge_base/rpc/trace_call.md`
- `docs/docs/users/knowledge_base/rpc/debug_trace_transaction.md`
- `docs/docs/developers/guides/trace_call_guide.md`

A pinned sha is the right instinct for a permalink, so if you would prefer a real commit rather than `main`, say which and I will use it.

**Not touching:** `CHANGELOG.md:1280` links the old `documentation/src/environment_variables.md`, which also 404s, but rewriting historical changelog entries would falsify the record.

Everything else came back clean: the other external links in the 67 markdown files resolve, and the relative links inside `docs/` that a naive check flags (`../../reference/cli#…`) are fine — `docs/docs/users/reference/cli.md` exists and Docusaurus resolves those.

AI disclosure: found with Claude Code (Claude Opus 5) sweeping the repo docs; I verified every path and the missing commit myself through the API before writing this, and trimmed it to the two things that actually need a decision.