How to structurize a multi-project documentation site? I'm currently working on our new Docs website. This site should hold the documentation for all our projects.

I thought I could structurize the content like this:

Project 1 (&lt;-- Section)
    Project section header 1 (&lt;-- Structure)
        Menu item 1.1
        Menu item 1.2
        Menu item 1.3
        Menu item 1.4
        Menu item 1.5
    Project section header 2
        Menu item 2.1
        Menu item 2.2
        Menu item 2.3

Project 2 (&lt;-- Section)
    Project section header 1 (&lt;-- Structure for Project 2)
        Menu item 1.1
        Menu item 1.2
        Menu item 1.3
        Menu item 1.4
        Menu item 1.5
    Project section header 2
        Menu item 2.1
        Menu item 2.2
        Menu item 2.3


I'd also like to have a dropdown which should list all available projects (Project 1, Project 2, ... from the above sample). If the visitor selects his desired project, a menu tree should be rendered in the sidebar with the corresponding structure.

Is this the right way?
Welcome to the Craft community. You try to create a new section for each project - right? In my opinion this is only necessary if you need different view and edit rights on each project. If you'r creating a "open for all" project documentation tool (open means for only one user group), than i would go with a single section. It's much easier to manage and starting with a new project. I would use two entry types in the section (https://craftcms.com/docs/sections-and-entries#entry-types). One entry type is for the project and one is for your chapters and the content.

    Example:
    1x Section called "projects"
        1x entry type called "project" with following fields (example)
            - name 
            - project description (rich text field)
            - project team (user field)
            - release date (table field with date and description)
            - ...

        1x entry type called "chapter" with following fields (example)
            - name/title
            - content (richt text field)
            - images (file field)
            - documents (file field)
            - ...

    Finaly you "project" section would be like this:
    Project 1 (entry type "project")
        - Chapter 1 (entry type "chapter")
           - Chapter 1.1 (entry type "chapter")
           - Chapter 1.2 (entry type "chapter")
        - Chapter 2 (entry type "chapter")
           - Chapter 2.1 (entry type "chapter")
           - ...
        - Chapter ...    

    Project 2 (entry type "project")
        - Chapter 1 (entry type "chapter")
           - ...      


For creating the dropdown-navigation, you can simple use the nav tag (https://craftcms.com/docs/templating/nav). Instead of a use option tags for output:

{% set entries = craft.entries.section('projects').level(1) %}

&lt;select id="nav" onchange="if (this.value) window.location.href=this.value"&gt;
    {% nav entry in entries %}
        &lt;option value="{{ entry.url }}"&gt;{{ entry.title }}&lt;/option&gt;
    {% endnav %}
&lt;/select&gt;


For the subnavigation you could take a look on this Craft recipe -> https://craftcookbook.net/recipes/fully-dynamic-hierarchical-nav-menu-template.

Other solutions:
Depending on how many content is used in each chapter, you could also make use of a matrix fields for your content...

Doe's this make sense for you?