Refine PR template

[waynexia]

This thread collects ideas to optimize our PR template

[waynexia]

Change the checklist to Q&A field. E.g.:

- Is necessary rustdoc comments writen?

- Is the changes covered by unit test / integration test?

Pros.:

• Do not have "task count" compare to checklist, which may be mixed with the checklist of PR itself.

Cons.:

• Not that eye-catching (after render)

[waynexia]

Tag out API-changing PR.

The major parts of document is in other repo greptimeteam/docs, it is easy to forget.

Don't know if it is possible to semi-automatic this, like the iuuse categories.

By the way, this can also be a question in the above suggestion.