Docs Update - Custom Boards

Hi there,

I'll take ownership of this one and formalise it a bit better.

This is relating to adding custom boards with extended name sets.

@peterpanstechland message:
in order to get the dts.ts.d autogenerated, you need to(this is what i did):

pxt clean
rm -Rf node_modules/pxt-common-packages
pxt link ../pxt-common-packages  (this is after you have modified the configkeys.h)
pxt buidaldts (if you get some error about missing file in the built folder, I got this error when i did this, and then I tried run "pxt builddaldts" in the pxt-maker/node_modules/pxt-common-package/ first, then run this command again in the pxt-maker folder)
this should update the dts.ts.d files.

Short Version (updated):

pxt clean
rm -Rf node_modules/pxt-common-packages
pxt link ../pxt-common-packages
pxt builddaldts
pxt serve --localbuild

Start by reviewing the issue's short-version command sequence for adding custom boards with extended name sets. Formalize the documented workflow around pxt clean, pxt link, pxt builddaldts, and pxt serve --localbuild; done means the custom-board process is clearly documented, though no documentation file is named here.