gh-1034: Expose glass.healpix module

[paddyroddy]

Description

Adds glass.healpix to the documentation similar to glass.arraytools.

Closes: #1034

Changelog entry

Added: The glass.healpix module to the documentation.

Checks

• Is your code passing linting?
• Is your code passing tests?
• Have you added additional tests (if required)?
• Have you modified/extended the documentation (if required)?
• Have you added a one-liner changelog entry above (if required)?

[paddyroddy]

This is annoying me