Add `aod` and `clearsky_index` to glossary

[RDaxini]

Test output is the cleanest it's been in a while. Docs build is also clean except for a few missing glossary items (ping @rdaxini):

sphinx/source/user_guide/modeling_topics/spectrum.rst:55: WARNING: term not in glossary: 'aod'
sphinx/source/user_guide/modeling_topics/spectrum.rst:63: WARNING: term not in glossary: 'clearsky_index'
sphinx/source/user_guide/modeling_topics/spectrum.rst:67: WARNING: term not in glossary: 'clearsky_index'

Of course those should be fixed, but not a blocker for the release IMHO. I'll try out a pre-release to ensure that the new Trusted Publishing arrangement (#2549) works. If all goes well then I'll make the real release.

Originally posted by @kandersolar in https://github.com/pvlib/pvlib-python/issues/2554#issuecomment-3329613783

Those warnings were fixed by removing the :term: sphinx role, but let's go back and add these terms to the glossary (and link any relevant instances of them in the docs)

Opening an issue before a PR to check people's opinions on definitions. Tentative suggestion to start:

aod: aerosol optical depth. Measure of aerosols (e.g., smoke particles, desert dust) distributed within a column of air from the instrument (Earth's surface) to the top of the atmosphere. [unitless]

(modified from source) Optionally, we could add a comment relating more directly to performance modeling, e.g. helps quantify atmospheric extinction / insight into spectral mismatch owing to selective scattering/absorption.

clearsky_index: clearsky index. Ratio of actual global irradiance to modeled clearsky global irradiance

Again, optional extra to add a comment explicitly relating the performance modeling implications. (modified from source)

[aman-coder03]

@RDaxini can i take this issue?