Add DP-GEN Structure.md to doc

[HuangJiameng]

A basic user guide.

[codecov-commenter]

Codecov Report

Merging #844 (a2ae0f1) into devel (40f66fd) will decrease coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##            devel     #844      +/-   ##
==========================================
- Coverage   36.08%   36.01%   -0.08%     
==========================================
  Files          97       97              
  Lines       17169    17279     +110     
==========================================
+ Hits         6196     6223      +27     
- Misses      10973    11056      +83     

Impacted Files	Coverage Δ
dpgen/generator/lib/make_calypso.py	70.06% <0.00%> (-7.46%)	⬇️
dpgen/generator/run.py	62.50% <0.00%> (-0.61%)	⬇️
dpgen/data/gen.py	47.88% <0.00%> (-0.60%)	⬇️
dpgen/generator/lib/run_calypso.py	10.00% <0.00%> (ø)
dpgen/generator/lib/calypso_run_opt.py	0.00% <0.00%> (ø)
dpgen/generator/lib/calypso_check_outcar.py	0.00% <0.00%> (ø)
dpgen/generator/lib/calypso_run_model_devi.py	0.00% <0.00%> (ø)
dpgen/generator/arginfo.py	99.25% <0.00%> (+0.05%)	⬆️
dpgen/generator/lib/abacus_scf.py	86.41% <0.00%> (+0.34%)	⬆️
dpgen/generator/lib/parse_calypso.py	84.84% <0.00%> (+0.47%)	⬆️

Help us with your feedback. Take ten seconds to tell us how you rate us.

[njzjz]

The file needs to be added to TOC in index.rst, otherwise it will not be shown in the website.

[HuangJiameng]

The file needs to be added to TOC in index.rst, otherwise it will not be shown in the website.

OK, it is added.