Add doc updates checklist to PR template

[inancgumus]

What?

Adds the documentation updates checklist to the PR template.

Why?

To streamline the release process and reduce mistakes.

Notes

• Ownership: The PR owner is responsible for making all documentation updates.
• Mandatory documentation updates: To prevent documentation from being overlooked, reduce mistakes, and streamline the release process (avoiding bottlenecks).
• Direct documentation updates: To reduce mistakes and avoid having to keep a to-do list throughout the cycle.
• External contributor PRs: The reviewers are responsible for updating the documentation (bonus: request updates from the contributor if possible).

[codebien]

➕ on my side, it would be good two split in two cases:

• I'm a k6 maintainer
• I'm an external contributor

[inancgumus]

@codebien 295e9c7

[olegbespalov]

Generally, LGTM, but I prefer to also include updating the labels of PR at that stage since as the release driver I'm pretty rely on them usually

[olegbespalov]

🚀