Update action in docs workflow

[prjemian]

GHA suggests:

docs (3.9)
This action is deprecated and no longer maintained. Please use mamba-org/setup-micromamba instead. See https://github.com/mamba-org/provision-with-micromamba#migration-to-setup-micromamba for a migration guide.

Originally posted by @prjemian in #282 (comment)

[prjemian]

Cannot drop conda entirely. gi package is needed or these errors arise:

File "/home/runner/work/hklpy/hklpy/docs/source/conf.py", line 15, in <module>
    import hkl
  File "/home/runner/work/hklpy/hklpy/hkl/__init__.py", line 31, in <module>
    gi.require_version("Hkl", "5.0")
  File "/opt/hostedtoolcache/Python/3.11.6/x64/lib/python3.11/site-packages/gi/__init__.py", line 1[26](https://github.com/bluesky/hklpy/actions/runs/6816901035/job/18539250311?pr=296#step:10:27), in require_version
    raise ValueError('Namespace %s not available' % namespace)
ValueError: Namespace Hkl not available