Add site_url in mkdocs.yml for broken stylesheet links in 404 page

[jeongukjae]

Pull Request Checklist

• Fixes 404 page's stylesheet links are broken #1142
• Tests added no test needed
• Documentation/examples added no additional documentation needed
• Good commit messages and/or PR title

Description of PR

Currently, 404 page is not rendered properly because stylesheet links are broken in there.
(details are described in the linked issue)
This PR adds site_url attribute in mkdocs.yml for the workaround of #1142 .

if css hash has changed, it can be broken again in 404 page of outdated docs.
but with this workaround, at least latest docs will be rendered properly.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.3%. Comparing base (d553546) to head (9bd3494).
Report is 116 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #1143     +/-   ##
=======================================
- Coverage   81.7%   81.3%   -0.4%     
=======================================
  Files         54      60      +6     
  Lines       4208    4720    +512     
  Branches     889    1000    +111     
=======================================
+ Hits        3439    3842    +403     
- Misses       574     647     +73     
- Partials     195     231     +36     

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