docs/howto: generating Go types from CUE definitions

[jpluscplusm]

Now that https://cuelang.org/docs/reference/command/cue-help-exp-gengotypes/ is available in a release, we should have a page that demonstrates a basic pass through its use.

The page doesn't need to be wholly minimal, in that it should probably include the use of optional features such as:

• the @go() attribute being used to rename and retype definitions and packages,
• across at least a couple of distinct CUE package directories, to show the creation of multiple cue_gen.go files. This complexity didn't read very well, on the page.

Suggested path: /docs/howto/generate-go-types-from-cue-definitions/.

[myitcv]

cc @mvdan

[jpluscplusm]

This should have been closed by cue-lang/cuelang.org@612ed7d.