Refined and renamed pull request template

[pasabanov]

Types of changes

• Configuration (GitHub pull request template)

Motivation

The previous pull request template had many rarely used headers and descriptive labels that would be better as comments.
Additionally, important types of changes were missing.

Description

1. Renamed the section PR Type to Types of changes and Describe changes to Description.
2. Added more types of changes.
3. Got rid of the checkboxes. Moved the list of types of changes into a comment.
4. Removed rarely used sections.
5. Moved some removed sections into a comment suggesting the addition of those sections.
6. Removed explanatory labels. Some of them were replaced with comments.
7. Added the label Related: .
8. Added extra blank lines under Types of changes and Description for easier text entry.
9. Renamed the template from PULL_REQUEST_TEMPLATE.md to pull_request_template.md.