Describe changelog types in developer docs

What does this MR do?

This adds the descriptions for the different changelog types from the previous bin/changelog script (see https://gitlab.com/gitlab-org/gitlab/-/blob/34af4653dfc4d84fc21ee68d7984a9fd59543a12/bin/changelog#L46-53).

Does this MR meet the acceptance criteria?

Conformity

• [-] I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• [-] I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• [-] I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.