Improve documentation on the testing organization in the repo

Not directly related to this PR, but I find the sheer number of places where tests can be run/written extremely confusing. We have tests inside the Alonzo impl, and we also have tests inside the `cardano-ledger-test` package. At the least, the CONTRIBUTING.md file would benefit from a guide explaining which tests should go where and why.

_Originally posted by @nc6 in https://github.com/input-output-hk/cardano-ledger/pull/3809#discussion_r1373271696_

Start with CONTRIBUTING.md and inspect the testing locations named in the issue: the Alonzo implementation and the cardano-ledger-test package. Document which tests belong in each location and why, so contributors can choose where to run or add tests. Done means the repository’s testing organization is explained clearly in CONTRIBUTING.md.

Written by the indexing model from the issue text.