docs: process principle improvements and anchors

• Improves verbiage of several process principles, such as avoiding self-merges or starting sprints with kick-off meetings. (closes #3659)

• Adds anchors to all process principles to make them easily citeable. Example: /processes/integrating.html#avoid-self-merges.

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>