📝 Update YAML introduction

[CollierCZ]

Why

Closes #2319

What's changed

• Split YAML intro into basic and PLatform-specific sections
• Updated for style and clarity
• Added plantuml diagrams
  • Added a server to the site would be difficult
  • The code for the diagrams is in the text and also in the SVGs themselves
  • So they can be updated at places like: https://www.plantuml.com/plantuml/uml/SyfFKj2rKt3CoKnELR1Io4ZDoSa70000

[github-actions]

Your Platform.sh environment has successfully deployed. 🚀

See the changed pages:

• https://yaml-intro-l5qs3nq-652soceglkw4u.eu-3.platformsh.site/create-apps/upgrading.html
• https://yaml-intro-l5qs3nq-652soceglkw4u.eu-3.platformsh.site/environments/default-environment.html
• https://yaml-intro-l5qs3nq-652soceglkw4u.eu-3.platformsh.site/languages/java/migration.html
• https://yaml-intro-l5qs3nq-652soceglkw4u.eu-3.platformsh.site/overview/yaml.html
• https://yaml-intro-l5qs3nq-652soceglkw4u.eu-3.platformsh.site/overview/yaml.html
• https://yaml-intro-l5qs3nq-652soceglkw4u.eu-3.platformsh.site/overview/yaml/platform-yaml-tags.html
• https://yaml-intro-l5qs3nq-652soceglkw4u.eu-3.platformsh.site/overview/yaml/what-is-yaml.html

[trolologuy]

I really like how plantuml has a much better look and feel and integrates better than other tools like mermaid.js (but is a tad harder to maintain).
It's a welcome addition and really helps to understand the flow and the logic!

There are a few typos. Other than that, the split / logic is much better than it was. Great PR!

[trolologuy]

✅

• Final built pages are working as expected.
• The information is correct.
• The information is comprehensive.
• The information is concise.
• The information is clear.