Deprecate the current Spectrum implementation

[vishwas-droid]

Following the discussion in #153, it seems that the current
Spectrum implementation may not be actively used and could
be removed in a future release.

This issue proposes introducing a formal deprecation step
using SunPy’s deprecation decorators before eventual removal.

If this direction looks good, I can prepare a PR to:

• Mark Spectrum as deprecated
• Emit an appropriate warning
• Update the documentation accordingly

Happy to proceed once there’s agreement on the expected
removal timeline.