text: Title: [DOC] Document less common in-memory data formats
  Body: 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](https://www.sktime.net/en/latest/api_reference.html)), 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:
  
  - [x] `SeriesXArray`
  - [ ] `SeriesDask`
  - [ ] `SeriesPolarsEager`
  - [ ] `SeriesGluontsList`
  - [ ] `SeriesGluontsPandas`
  - [ ] `PanelDask`
  - [ ] `PanelPolarsEager`
  - [x] `PanelGluontsList`
  - [ ] `PanelGluontsPandas`
  - [ ] `HierarchicalDask`
  - [ ] `HierarchicalPolarsEager`
  - [x] `TablePdDataFrame`
  - [x] `TablePdSeries`
  - [x] `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!