CI: Merge two docs CIs to one

[Zeroto521]

PR Description

Please describe the changes proposed in the pull request:

• Merge docs-preview.yml into docs.yml. To simplify duplicated CIs.
• Add documentation mark for pytest. documentation related test cases will be tested when mkdocs package is install in environment. To fix [INF] Set multi-test env to compat dependency and system #1143

This PR resolves #(put issue number here, and remove parentheses).

PR Checklist

Please ensure that you have done the following:

1. PR in from a fork off your branch. Do not PR from <your_username>:dev, but rather from <your_username>:<feature-branch_name>.

2. If you're not on the contributors list, add yourself to AUTHORS.md.

3. Add a line to CHANGELOG.md under the latest version header (i.e. the one that is "on deck") describing the contribution.
   • Do use some discretion here; if there are multiple PRs that are related, keep them in a single line.

Automatic checks

There will be automatic checks run on the PR. These include:

• Building a preview of the docs on Netlify
• Automatically linting the code
• Making sure the code is documented
• Making sure that all tests are passed
• Making sure that code coverage doesn't go down.

Relevant Reviewers

Please tag maintainers to review.

• @ericmjl

[Zeroto521]

two same names are kind of duplicated

[codecov]

Codecov Report

Merging #1183 (98ed58e) into dev (f0c7906) will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##              dev    #1183      +/-   ##
==========================================
+ Coverage   97.93%   97.96%   +0.02%     
==========================================
  Files          76       76              
  Lines        3387     3387              
==========================================
+ Hits         3317     3318       +1     
+ Misses         70       69       -1     

[ericmjl]

Thank you, @Zeroto521! Astute eyes and great work figuring out how to simplify stuff!