Cleanup bootstrap `rust-docs` dist component handling

[jieyouxu]

This is a long-standing cleanup that needs to be done eventually:

• Get rid of doc_out -- using the same shared directory for multiple doc steps is wrong, there should be an explicit "build separately but eventually merge together" step to prevent any possibility of doc step artifacts stepping over each other.
• Do not rely on run_default_doc_steps which is way too implicit; instead, explicitly enumerate the "default steps" that should be included as part of the "preset" Doc step.

For the "implicit" part, note that we use this logic for both rust-docs dist component and rustc-docs dist component.

For the "explict" part, we also ensure them in:

• test::Linkcheck
• test::HtmlCheck

(Needs further investigation, there are some convoluted recursive logic)