Document internal_compact:: name space for arma::fill

[shrit]

Hi,

This issue is a place holder so we do not forget about documenting internal_compact namespace as a wrapper for the fill struct.
Probably somewhere in the developer guide.
More details will be in the upcoming PR.

[rcurtin]

Yes, actually, it would be really nice to have some developer_guide.md or similar file that has this information and then other relevant things (like the style guide) from the Wiki. We could add it to the sidebar and then it should be easy to point people there.

[mlpack-bot]

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