docs: add important note

[lauthieb]

Changes description

Add the important note to indicate that this current version of Vitamin will no longer evolve and only accept bug fixes, new icons, and assets from now on. It also deletes the feature_or_improvement_request.md & new_component_request.md ISSUE_TEMPLATES.

(cf. https://github.com/Decathlon/vitamin-design/pull/225)

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 related showcases.
• [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