build additional separate indices for named entities

[prjemian]

It would be nice to have ...

• a separate index of all the NXDL class names
• a separate index of all field names
• a separate index of all attribute names

[prjemian]

suggestion that might help: http://stackoverflow.com/questions/26672918/how-do-i-reference-a-custom-index-generated-in-my-own-sphinx-extension

[prjemian]

• http://www.sphinx-doc.org/en/stable/extdev/appapi.html?highlight=custom%20%20index
• http://www.sphinx-doc.org/en/stable/domains.html?highlight=custom%20%20index

[prjemian]

• https://doughellmann.com/blog/2010/05/09/defining-custom-roles-in-sphinx/

[prjemian]

Related to the new (PR #1099) vocabulary to be posted in the web site