feature: add frequency_at_index method to GenericSpectrogram (#129).

[dhanushsaimudari]

Added the frequency_at_index method to the GenericSpectrogram class. This method allows users to retrieve the physical frequency value associated with a specific array index along the frequency axis.

The implementation includes:

-Validation logic to ensure the provided index is within the bounds of the frequency axis.
-Raising a descriptive IndexError if the index is out of range, helping users debug data mapping issues.

AI Assistance Disclosure

AI tools were used for :

[x] Code generation (Initial drafting of the method structure)
[x] Research and understanding (Clarifying rST syntax for the changelog)

Note: I have manually verified the logic using a local test suite with dummy spectrogram data to ensure the mapping is accurate.

Related Issues
Closes #129

[samaloney]

Thanks for the PR just a few housekeeping issues please retain and use the PR template which is created when the PR is opened. (You can see it here: https://github.com/sunpy/.github/blob/main/.github/PULL_REQUEST_TEMPLATE.md)

Also, please note, as discussed on the #129, this isn’t really ready for work until after the decision on the underlying data structure for the Spectrogram object.

[dhanushsaimudari]

Thank you for the feedback! @samaloney I have updated the PR description to follow the official template. I understand that the data structure decision for the Spectrogram object is still pending; I'm happy to keep this PR as a draft or wait for the decision before making further adjustments.

Also I'm looking at the background subtraction task. Are we prioritizing a specific algorithm like constant-median subtraction for the initial implementation, or are we more focused on the pluggable API structure first?