(docs): add Jest and ESLint subsections to Customization

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 https://github.com/jaredpalmer/tsdx/issues/692#issuecomment-616745580 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

jaredpalmer / tsdx