Menu navigation upgrade

[alan-cugler]

The menu tree on the left side needs to be expandable for searching for a specific page.

Example need case: I am hunting for the attachments page that's nested in the Data Modelling menu tree, but I can't see it unless I click on the Data Modeling page from the menu tree.

It's not a huge annoyance at the moment because most stuff seems to only be nested only once under a parent page. However, as Lektor grows and more features, depth, and complexity blooms, no doubt so will that menu tree. At some point it will become difficult for new curious people who want to try Lektor, because the documentation will become to time consuming trying to comb through the nested menu for the specific feature they are struggling with.

[goanpeca]

Makes sense :+1: