Update "Build a dApp in 15 minutes" page - grammar, removed redundant text, updated diagrams

[andrei-x1]

Description of the pull request (what is new / what has changed)

Update "Build a dApp in 15 minutes" page:

• grammar
• removed redundant text
• redesign 3 diagrams

Removed external branch reference from the PR template because it looks deprecated (the current README file focuses on the development branch for all users)

Did you test the changes locally ?

• [x] yes
• [ ] no

Which category (categories) does this pull request belong to?

• [ ] document new feature
• [x] update documentation that is not relevant anymore
• [ ] add examples or more information about a component
• [x] fix grammar issues
• [x] other

[andrei-x1]

Please review @bogdan-rosianu , @ssd04 .