Book shows accumulator API as `::push()` instead of `.accumulate()`

[joshuadavidthomas]

The book's overview section shows this syntax for accumulators:

Diagnostics::push(db, "some_string".to_string())

But this gives a "no function or associated item named push found" error. The actual API uses:

Diagnostics("some_string".to_string()).accumulate(db)

All the examples and tests use .accumulate(db), not ::push(). The incorrect syntax appears in book/src/overview.md, dating back to this commit from August 2022.

Is ::push() a planned API change, or is this just outdated documentation? Happy to open a PR to fix it if it's the latter.

The discrepancy caused some confusion when following the book - took a bit to figure out the correct API.