[MNGSITE-484] On welcome page, add "Learn" to "Use", "Extend", "Contribute"

**[Thorbjørn Ravn Andersen](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=ravn)** opened **[MNGSITE-484](https://issues.apache.org/jira/browse/MNGSITE-484?redirect=false)** and commented

[The welcome page](https://maven.apache.org/index.html)is likely the first a programmer unfamiliar with Maven sees when looking for official documentation.  There is currently "Use", "Extend" and "Contribute".

I think it would be beneficial to have a "Learn" entry first with suitable entries. 

One could be "What makes Maven different from most other buildtools?" (or something like that) because understanding the lifecycle is important even for experienced programmers.

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No further details from [MNGSITE-484](https://issues.apache.org/jira/browse/MNGSITE-484?redirect=false)

Start with the welcome page at https://maven.apache.org/index.html and inspect how the existing “Use”, “Extend”, and “Contribute” entries are defined. Decide on suitable “Learn” entries, including material explaining Maven’s lifecycle, and consider the work done when “Learn” appears first with useful links and wording consistent with the other entries.

Written by the indexing model from the issue text.