add mermaid reference, link to docs:

[edasmalchi]

Description

Simple docs edit to add reminders and links to mermaid in both the portfolio and docs contribution sections.

Resolves #[issue]

Type of change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [x] Documentation

How has this been tested?

Post-merge follow-ups

• [x] No action required
• [ ] Actions required (specified below)