Add description of tiers to the contribution guide

[darkwisebear]

Introduce the tiers to the guideline documentation

• Add a description of the three tiers that we identified to https://github.com/PLeVasseur/safety-critical-rust-coding-guidelines/blob/main/CONTRIBUTING.md
• Update the tag description and prefill text at https://github.com/PLeVasseur/safety-critical-rust-coding-guidelines/blob/main/.github/ISSUE_TEMPLATE/CODING-GUIDELINE.yml
• Inspect guidelines for possible missing tags and create a PR with the links in place