Improve docval for IndexSeries init

[rly]

Motivation

In coordination with NeurodataWithoutBorders/nwb-schema#644

The goal is to not write resolution, conversion, and offset for IndexSeries.

This PR also updates IndexSeries.__init__ so that unit cannot be passed in, since the schema requires a fixed value of "N/A" and currently in PyNWB, passing any different value will result in an error. This change improves the usability -- users do not need to provide `unit="N/A" during construction.

WIP. Removing resolution, conversion, and offset for IndexSeries.__init__ does not work because when resolution=None is passed to TimeSeries.__init__, then resolution is set to the default value for resolution (-1) specified in TimeSeries.

How to test the behavior?

Show how to reproduce the new behavior (can be a bug fix or a new feature)

Checklist

• Did you update CHANGELOG.md with your changes?
• Have you checked our Contributing document?
• Have you ensured the PR clearly describes the problem and the solution?
• Is your contribution compliant with our coding style? This can be checked running ruff check . && codespell from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.

[rly]

We will punt this to the schema 2.10 release.