Fix pull request template's markdown, reduce the required content

[tormi]

The current pull request template has too many elements and content. Let's try to reduce it to a reasonable level.

Also, there are several syntax issues with the pull request template markdown:

[tormi]

We should probably introduce the markdown validation check, too.

[sampoturve]

I wonder whether the "Best practices" section is something that should be rethinked (or removed even), as it currently only provides a summary for accessibility. There are basically million best practices one could list here, and like the accessibility things, they should already be required on the definition of done of everything.

[tormi]

I totally agree. PR template should be really minimal, precise and not an obstacle as it's now (at least for me).
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