[9.20] new: doc: Provide guidelines for tool-generated content

[9.20] new: doc: Provide guidelines for tool-generated content

In the last few years, the capabilities of coding tools have exploded.
As those capabilities have expanded, contributors and maintainers have
more and more questions about how and when to apply those capabilities.

Add new documentation to guide contributors on how to best use BIND 9
development tools, new and old.

In short: Please show your work and make sure your contribution is
easy to review.

This has been adopted from the Linux Kernel guidelines.

Backport of MR !11447

Merge branch 'backport-ondrej/clarify-the-use-of-tools-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11581