chore: add `documentation_improvement.md` issue template

[lauthieb]

Checklist

• [x] Make sure you are requesting to pull a topic/feature/bugfix branch. Please, don't request directly from your main!
• [x] Check commits & PR names matches our requested structure. It must follow the https://www.conventionalcommits.org pattern.
• [x] Check your code additions will fail neither code linting checks.
• [x] I have reviewed the submitted code.
• [x] I have tested on an iPhone device/simulator.
• [x] I have tested on an iPad device/simulator.
• [x] If it includes design changes, please ask for a review design-system-core-team-design GitHub team.

Does this introduce a breaking change?

• No

[sonarcloud[bot]]

Kudos, SonarCloud Quality Gate passed!   

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