add doc: create a model

[njzjz]

No description provided.

[amcadmus]

The links are broken in markdown

[njzjz]

The links will be parsed in sphinx. I have no idea how to make it also work in GitHub, so I change to {py:class}deepmd.descriptor.descriptor.Descriptor which will also generate links in sphinx.

[codecov-commenter]

Codecov Report

Merging #1143 (a0a603a) into devel (76bc05c) will decrease coverage by 11.37%.
The diff coverage is n/a.

❗ Current head a0a603a differs from pull request most recent head 00d0ce3. Consider uploading reports for the commit 00d0ce3 to get more accurate results

@@             Coverage Diff             @@
##            devel    #1143       +/-   ##
===========================================
- Coverage   75.66%   64.28%   -11.38%     
===========================================
  Files          88        5       -83     
  Lines        7034       14     -7020     
===========================================
- Hits         5322        9     -5313     
+ Misses       1712        5     -1707     

Impacted Files	Coverage Δ
deepmd/loss/ener.py
deepmd/entrypoints/convert.py
deepmd/utils/data.py
source/op/_tabulate_grad.py
deepmd/descriptor/se_a.py
deepmd/cluster/__init__.py
deepmd/utils/graph.py
deepmd/infer/deep_pot.py
deepmd/infer/__init__.py
deepmd/utils/type_embed.py
... and 69 more

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