Initial version intro page Structure section

[kjellp]

Describe the pull request:

• [ ] Bug fix
• [ ] Functional change
• [ ] New feature
• [ ] Code cleanup
• [ ] Build system change
• [x] Documentation change
• [ ] Language translation

Drafted a new high level intro page for the Structure section, and linked it into the left menu.

Mentions:

@norling @lilachic @tuzov