Update PR template

[david-yz-liu]

Proposed Changes

(Describe your changes here. Also describe the motivation for your changes: what problem do they solve, or how do they improve the application or codebase? If this pull request fixes an open issue, use a keyword to link this pull request to the issue.)

This pull request makes a few changes to the current pull request templates.

1. Merge the "Motivation and Context" and "Description of Changes" sections, since those were often blurred.
2. Add explicit places for screenshots and a link to the documentation repository.
3. Change the "type of change" from a task list into a table, to avoid having to delete unnecessary items.
4. Add two new change types, "Dependency" and "Internal".
5. Rework the "checklist" so that all items are required to be checked, but provide a few details about when certain ones apply or not.
6. Add new instructions, and make them visible on the template rather than just in comments.

I also updated the Changelog structure to include subheadings.

Screenshots of your changes (if applicable)

Associated [documentation repository](https://github.com/MarkUsProject/Wiki) pull request (if applicable)

Type of Change

(Write an X or a brief description next to the type or types that best describe your changes.)

Type	Applies?
🚨 Breaking change (fix or feature that would cause existing functionality to change)
✨ New feature (non-breaking change that adds functionality)
🐛 Bug fix (non-breaking change that fixes an issue)
♻️ Refactoring (internal change to codebase, without changing functionality)
🚦 Test update (change that only adds or modifies tests)
📦 Dependency update (change that updates a dependency)
🔧 Internal (change that only affects developers or continuous integration)	X

Checklist

(Complete each of the following items for your pull request. Indicate that you have completed an item by changing the [ ] into a [x] in the raw text, or by clicking on the checkbox in the rendered description on GitHub.)

Before opening your pull request:

• I have performed a self-review of my changes.
  • Check that all changed files included in this pull request are intentional changes.
  • Check that all changes are relevant to the purpose of this pull request, as described above.
• I have added tests for my changes, if applicable.
  • This is required for all bug fixes and new features.
• I have updated the project documentation, if applicable.
  • This is required for new features.
• If this is my first contribution, I have added myself to the list of contributors.

After opening your pull request:

• I have updated the project Changelog (this is required for all changes).
• I have verified that the pre-commit.ci checks have passed.
• I have verified that the CI tests have passed.
• I have reviewed the test coverage changes reported by Coveralls.
• I have requested a review from a project maintainer.

Questions and Comments

(Include any questions or comments you have regarding your changes.)

[coveralls]

Pull Request Test Coverage Report for Build 8903652909

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 91.475%

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Totals
Change from base Build 8862784121:	0.0%
Covered Lines:	39849
Relevant Lines:	42897

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💛 - Coveralls