(docs): add Jest and ESLint subsections to Customization

[agilgur5]

• while the docs do say these are run fairly directly, recently folks
  seem to be particularly confused about how to configure Jest

  • so explicitly mention jest.config.js and .eslintrc.js to make
    that a bit more clear
    • don't mention package.json options as I think that should be
      implicitly implied enough now
      • and because ESLint config works a bit differently in the two
        cases (see existing issue)
  • and more explicitly say that all tsdx test flags are forwarded to
    Jest

• also add links to "its own config" for each of the Babel, Jest, and
  ESLint subsections so people can look at the source quickly if they
  want

  • the Babel one is a bit harder to read though

• and specify the merge behavior for each, i.e. shallow or deep

  • and the ordering when it comes to Babel, as order matters there

---

To help with issues like #695 and #692 (comment) as I guess it's not clear enough that you can do this already
Also specifies merge behavior of Babel since #543 is a thing

[agilgur5]

LGTM