Update navigation for User and Plugin Developer Centres: Maven 3 vs Maven 4

[sparsick]

Based on the proposal in #726

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[sparsick]

Currently, I changed the structure of the navigation menu. I could also reorder the sources based on that structure. What do you think?

[hboutemy]

trying to make the update clearly visible before any discussion:

• Users Centre menu
  before / after =
  

• Plugin Developer Centre:
  before / after =
  

[hboutemy]

I could also reorder the sources based on that structure

in general, we avoid moving files to not loose search engines indexing

[hboutemy]

on "Plugin Developer Center":
I think it's too early to have that prominent "Maven 3" section
but immediately adding a "Maven 4" entry to starting introducing Maven 4-specific plugins topic (using experimental Maven 4 API that will be provided in Maven 4.0.0) would be a good idea to start communicating reasonably about it
(and re-explain that Maven 3 plugins are expected to work in Maven 4, no hurry: future is not yet now)

[hboutemy]

on "User Center":

• introduction of "Maven Build Fundamentals" and "Maven Build Config Fundamentals": splitting in 2 groups looks an interesting idea
• introduction of "Maven 3" entry with the "Getting Started Guide" (and all its content": I'm not convinced this is the right time for this "Maven 3" entry, and I'm not convinced at all about the content