Docs cleanup (IS portion)

[kdayday]

• add docstrings for types that were missing in PSY (e.g., TimeSeriesKey)
• fix broken reference links
• update see also links for timeseries interface functions

Required for PSY docs to successfully compile in documenter 1.0 in NREL-Sienna/PowerSystems.jl#1176

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.28%. Comparing base (f6de7fd) to head (fefd03a).
Report is 8 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #394   +/-   ##
=======================================
  Coverage   78.28%   78.28%           
=======================================
  Files          71       71           
  Lines        5484     5484           
=======================================
  Hits         4293     4293           
  Misses       1191     1191           

Flag	Coverage Δ
unittests	78.28% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/production_variable_cost_curve.jl	83.72% <ø> (ø)
src/static_time_series.jl	33.33% <ø> (ø)
src/system_data.jl	91.28% <ø> (ø)
src/time_series_interface.jl	95.58% <ø> (ø)
src/time_series_storage.jl	61.53% <ø> (ø)
src/time_series_structs.jl	92.85% <ø> (ø)
src/utils/logging.jl	91.50% <ø> (ø)
src/utils/recorder_events.jl	92.23% <ø> (ø)
src/value_curve.jl	93.00% <ø> (ø)