Various documentation issues

[Elias-Graf]

Seems like the anchor referenced here:

https://github.com/salsa-rs/salsa/blob/88a1d7774d78f048fbd77d40abca9ebd729fd1f0/book/src/tutorial/debug.md?plain=1#L25C1-L29C4

is missing from examples/calc/ir.rs. I sadly don't have a clue how any of this works, otherwise I would have made a PR :D.

[Elias-Graf]

A few more pages that seem to be missing content:

• https://salsa-rs.github.io/salsa/tutorial/checker.html#defining-the-checker
  
• https://salsa-rs.github.io/salsa/tutorial/interpreter.html#defining-the-interpreter
  
• https://salsa-rs.github.io/salsa/plumbing/query_ops.html#query-operations
  
• https://salsa-rs.github.io/salsa/plumbing/maybe_changed_after.html#maybe-changed-after
  
• https://salsa-rs.github.io/salsa/plumbing/fetch.html#fetch
  

[Elias-Graf]

https://salsa-rs.github.io/salsa/tutorial/accumulators.html#defining-the-parser-reporting-errors

should be something like:

        Diagnostic(format!(
            "Error in file {}: {:?}\n",
            file.path(db)
                .file_name()
                .unwrap_or_else(|| "<unknown>".as_ref())
                .to_string_lossy(),
            error,
        ))
        .accumulate(db);