[DOC] Document less common in-memory data formats

The recent PR https://github.com/sktime/sktime/pull/7231 adds an API reference page for in-memory data representation.

With this, detailed documentation on the most commonly used time series machine types were also added.

However, documentation for the less common formats is still missing - we should add this.

Recipe:

1. pick one type and help document it! (one pull request per type). See below for a list of undocumented types.
2. use get_examples from sktime.datatypes to generate some examples, and look at MTYPE_REGISTER for a short description. You can also use convert_to to see how already documented types convert to the documented one.
3. add a docstring to the class in one of the _check modules, e.g., sktime.datatypes._panel._check for one of the Panel mtypes. The name of the class is the same as below. All modules already contain complete examples, so "follow the pattern".

To identify a missing case, see the list below.

Alternatively, you can also:

• search for classes with empty docstrings in one of the datasets _check modules, within the _series, _panel, _hierarchical or _table submodules.
• look at the "Data Format Specifications" page on the API reference (link), click through concrete representations and search for empty docstrings.

Docstrings should be completed with a description of the in-memory type. Example descriptions can be found for the more common types, in the API reference, or the aforementioned modules.

Classes currently empty:

• SeriesXArray
• SeriesDask
• SeriesPolarsEager
• SeriesGluontsList
• SeriesGluontsPandas
• PanelDask
• PanelPolarsEager
• PanelGluontsList
• PanelGluontsPandas
• HierarchicalDask
• HierarchicalPolarsEager
• TablePdDataFrame
• TablePdSeries
• TableNp1D
• TableNp2D
• TableListOfDict
• TablePolarsEager

FYI @Abhay-Lejith and @pranavvp16, as you wrote some of the respective plug-ins - helping with the docs would be much appreciated, since you know most about these!