fix: do the deep clone in a GH action, "doc-gen"

[ryo-takahashi-1206]

Summary

1. Overview

• As shown in this run, "PR docs: remove version-hardcofing from Sphinx #128" causes a build break
• I confirmed actions/checkout in "doc-gen" does only a shallow clone, which fetch no git tag

2. Implementation details

• Change actions/checkout to the deep clone in "doc-gen"
• Not to repeat this stupid mistake, I propose to trigger "doc-gen" also at pre-merge (i.e. on.pull_request)
• However, to avoid messy git push in GH pages, I introduce a skip condition depending on github.ref

3. Future Work

• Fold per-tag Sphinx docs into the single GH pages like other projects

This change is 

[tk-woven]

Ah, we did have the tags, nice catch. Thank you!

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @ykkawana-woven)

[tk-woven]

*didn't have

Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @ykkawana-woven)