Create page about internal datasets in documentation

[ostreech1997]

Before submitting (must do checklist)

• Did you read the contribution guide?
• Did you update the docs? We use Numpy format for all the methods and classes.
• Did you write any new necessary tests?
• Did you update the CHANGELOG?

Proposed Changes

Closing issues

Closes #168.

[github-actions]

🚀 Deployed on https://deploy-preview-175--etna-docs.netlify.app

[codecov]

Codecov Report

Attention: 38 lines in your changes are missing coverage. Please review.

Comparison is base (dd1ace5) 88.93% compared to head (cb1997c) 89.02%.
Report is 4 commits behind head on internal_datasets.

Files	Patch %	Lines
etna/analysis/forecast/plots.py	42.85%	20 Missing ⚠️
etna/datasets/internal_datasets.py	84.61%	6 Missing ⚠️
etna/models/nn/deepar_native/loss.py	94.33%	3 Missing ⚠️
etna/datasets/tsdataset.py	96.07%	2 Missing ⚠️
etna/models/base.py	66.66%	2 Missing ⚠️
etna/pipeline/hierarchical_pipeline.py	91.66%	2 Missing ⚠️
etna/transforms/decomposition/deseasonal.py	83.33%	1 Missing ⚠️
etna/transforms/math/limit.py	97.82%	1 Missing ⚠️
etna/transforms/timestamp/event.py	98.33%	1 Missing ⚠️

Additional details and impacted files

@@                  Coverage Diff                  @@
##           internal_datasets     #175      +/-   ##
=====================================================
+ Coverage              88.93%   89.02%   +0.09%     
=====================================================
  Files                    205      199       -6     
  Lines                  13026    13171     +145     
=====================================================
+ Hits                   11585    11726     +141     
- Misses                  1441     1445       +4     

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