Add description for typescript facet and build script

[daogrady]

1. Adds a description for how to use cds add typescript
2. Adds a description for using cds build with TypeScript (which replaces the steps that formerly had to be done manually)
3. Updates the output for cds-typer --help

[LotharBender]

@daogrady - outDir seems to match the default value and might not need to be configured. Reading the
value from the cds configuration might even be more robust:
path.join(cds.env.build.target, cds.env.folders.srv)

[daogrady]

🤔 I don't think we can dynamically add values here, as this is the recommendation for the basic layout of a tsconfig.json, which is a static file.

[LotharBender]

I thought the typescript build task could calculate the default path if outDir isn't set.
Of course, this is only an option if outDir isn't used by the typescript compiler itself which seems to be the case.

[daogrady]

Yes, you are correct about that! 🙂
But this part of the documentation is about a tsconfig.json the user could manually create to configure how the build task works, hence the preamble "We recommend the following basic setup for such a file:".