Fix documentation of NXaperture

[phyy-nx]

• NXaperture was marked as deprecated but it is not, so fix the documentation
• Explicitly add an unnamed NXoff_geometry field, the existence of which was implied by the GEOMETRY deprecation

Fixes #1231

[phyy-nx]

Hi all, reviewing NXaperture more today I noticed that the NXtransformations and (deprecated) NXgeometry fields are unnamed, so I thought it reasonable that we add NXoff_geometry as unnamed as well, instead of using the name SHAPE like we did on the Telco. Thoughts? @mkoennecke

Also, it's possible since the deprecated field NXgeometry implies that there should be an NXoff_geometry field, that this PR is only a documentation fix, and therefore doesn't require a vote:

GEOMETRY: (optional) NXgeometry DEPRECATED: Use the field depends_on and NXtransformations to position the aperture and NXoff_geometry to describe its shape

I'll wait for any feedback on this PR, but unless I hear otherwise, I'll start a vote tomorrow.

[phyy-nx]

Hello, please vote by providing an emoji on this comment. Thanks.

[phyy-nx]

Vote has passed. Needs one approving review before it can be merged @nexusformat/developers. Thanks.