Document the new navigation page block

[tjerman]

The documentation should go here (reference) and here (configuration)

• [x] Add to the page block reference (https://github.com/cortezaproject/corteza-docs/blob/2023.3.x/src/modules/integrator-guide/pages/compose-configuration/page-blocks.adoc, https://docs.cortezaproject.org/corteza-docs/2022.9/integrator-guide/compose-configuration/page-blocks.html). Use past page blocks as an example on how it should look like, what contents to write.

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• [x] Provide example -- add a few examples that come to mind, no need to go overboard (https://github.com/cortezaproject/corteza-docs/tree/2023.3.x/src/modules/integrator-guide/pages/compose-configuration/examples, https://docs.cortezaproject.org/corteza-docs/2022.9/integrator-guide/compose-configuration/examples/index.html)
• Make a new file
• include it in the index.adoc https://github.com/cortezaproject/corteza-docs/blob/2023.3.x/src/modules/integrator-guide/pages/compose-configuration/examples/index.adoc
• include it in the nav.adoc https://github.com/cortezaproject/corteza-docs/blob/2023.3.x/src/modules/integrator-guide/nav.adoc

Perhaps take a quick look here for some inspiration.

Original issue https://github.com/cortezaproject/corteza/issues/449