Remove the separate "Install" menu item

[matkuliak]

Summary of the changes / Why this is an improvement

Checklist

• [ ] Link to issue this PR refers to (if applicable):
• [x] CLA is signed

[matkuliak]

Hi @amotl. This is one of the possible solutions for the "duplicate installation link"

I could use your advice here. There's not many ways to solve this, right?

• One solution is this PR, remove the standalone link to "Installation" that's the second link in unrolled menu at the moment. It reduces the discoverability somewhat but is probably the easiest solution.
• Keep the current standalone link, but hide it ("it" as the current second menu item) when the "Installation" page is active.

Any better solution that you can see? Thanks!

[amotl]

Thank you.