Organize sections into a repositories chapter within navigation.

[gemmaro]

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Hello,

This is a modification to add a "Repositories" chapter to the navigation.
Currently, at the beginning of the JPA chapter, there is a link to "Working with Spring Data Repositories".
However, since this page is not in the navigation, readers might not know where to go next.
Therefore, I have organized the sections related to repositories into a single chapter.

Thank you.

[mp911de]

The structure of JPA is very different from e.g. MongoDB as JPA doesn't have a general section. With the Antora-based navigation we intentionally merged the Commons-part with the store-specific one as previously that split has caused quite some confusion.

That being said, if you have ideas about refining the structuring while keeping the merged approach, feel free to rework your pull request.