[dub-guide/building] Basic content

[x] talk about dub build + dub run
[ ] talk about configurations and build types
[ ] link to dub test
[ ] example how to build and run things
- [ ] show how to pass arguments to the app directly from the CLI using --
[ ] talk about how dependencies are built, link to dependency docs
[ ] talk about where builds are stored
[ ] mention that builds are cached
[ ] mention that builds are hard-linked from the .dub directory
[ ] mention about overriding compiler and architecture
[ ] mention --single mode / link to it

dlang / dub-docs