DOC: add table showing diffractometer geometries and relation to SPEC

[prjemian]

Users have asked for how to transition from SPEC to bluesky's hklpy. A table of the diffractometer geometries should be easy to find. It's not yet. (Different than #246)

[prjemian]

Here's a fragment of a table I collected some years ago. The most common geometry was E4CV/fourc (representing ~60% of diffractometers counted at APS at the time of the survey).

Since then, I believe (must confirm) that the Petra3_p09_eh2 geometry has worked for a psic diffractometer (in the survey, ~20% of diffractometers at APS).

[prjemian]

Add as table to the SPEC command cross-reference page.

Also link from the geometries page. That page is created by this code.