Fix build-doc error

[nptdat]

Problem

• Pushing static documents built by mkdocs raises error if the branch gh-pages exists

Cause

• In github action, all of existing branches including gh-pages are not checkout.
• So, mkdocs puts documents to the new gh-pages and push to github.
• Github rejects the push to existing gh-pages

Solution

• Simply checkout all existing branches in github action (build-doc.yaml)

Results

• See the build-doc results in the 3 commits, including re-producing & fixing the problem.