Update `pull_request_template.md`

[ParadoxV5]

• ~~The Jira issue number for this PR is: MDEV-37908~~

Description

• Replace the KB link with a Docs link
  • This is the main motivation for these changes.
• Improve the grammar and wording
  • Mainly so my grammar checker would stop complaining about basic-tier mistakes that I didn’t make.
• Move the section comments below their section headings
  • They explicitly belong to their sections in this format.

Release Notes & How can this PR be tested?

N/A

Basing the PR against the correct MariaDB version

• [x] This is a new feature or a refactoring, and the PR is based on the main branch.
• [x] This is a bug fix, and the PR is based on the earliest maintained branch in which the bug can be reproduced.

  • You must create templates on the repository's default branch. Templates created in other branches are not available for collaborators to use. ⸺ About issue and pull request templates § Pull request templates

PR quality check

• ~~I checked the CODING_STANDARDS.md file and my PR conforms to this where appropriate.~~
• [x] For any trivial modifications to the PR, I am ok with the reviewer making the changes themselves.

[svoj]

Frankly speaking I'd remove this template. People don't seem to bother following it in many cases. And it doesn't seem to bring us much value, at the cost of annoying those who create pull requests often. I'd rather have a link to PR guidelines and the let PR authors fill description in a free form.

[vuvova]

I agree with @svoj , let's remove it

[ParadoxV5]

Fine by me!

Though the “checklist” was sometimes helpful. Let’s move them to https://mariadb.org/get-involved/getting-started-for-developers/submitting-pull-request/ or even CONTRIBUTING.md.

[gkodinov]

Please check (and hopefully review) https://github.com/MariaDB/server/pull/4487 about the removal of the file. If you find it appropriate please close this one in its favor.