fix: add font-awesome css to docsify index.html

[heyfirst]

📑 Summary

Because docsify index.html does not have font-awesome CSS file setup yet. Thus, Docsify can not render the font-awesome icon in any example pages.

This PR is adding font-awesome.min.css in version 5.9.0 and also adding statement to the document that Mermaid now only support font-awesome version 4 and 5 only. The reason explained on 📏 Design Decisions

Resolves #3190

📏 Design Decisions

This PR is adding font-awesome.min.css in version 5.9.0 (Latest Version: 6.1.2) because Mermaid's font-awesome seems supported only old syntax.

For example,

• (Twitter icon) in version 6 is using fa-brands prefix instead of fab, such as <i class="fa-brands fa-twitter"></i>
• (Car icon) in version is also using fa-solid as well, such as <i class="fa-solid fa-car"></i>

We should update the font-awesome extractor to support the version 6 as well.

But it should not included in this PR as it out of scope of the issue. We need to update regex to extract the right fa class and write a unit test to make sure every versions (v4, v5, and v6) are working properly.

📋 Tasks

Make sure you

• 📖 have read the contribution guidelines
• 💻 have added unit/e2e tests (if appropriate)
• 🔖 targeted develop branch

🚨 The unit test fail because of latest commit on develop branch. 8681e78

[knsv]

Thanks, long overdue!