Context-Aware Sub-Menu Rendering

[ppaulweber]

This PR introduces the following changes:

• introduces generic context-aware sub-menu rendering e.g. for the ASM method with explicit References entry to the BibTex entries as an example for now (see Figure 1)
• can be used an every page level and can be configured in the menus.toml file (see 31a0138)
• closes: add submenu for methods #17

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Figure 1: Example sub-menu rendering for the ASM method

[araschke]

I noticed that when using the submenu, the highlighting of the currently selected main menu disappears. I think a user might be less lost if this is not the case.