Automatically rebuild HTML when templates or YAML update about doc-pipeline

[tbpg]

Sometimes, it's nice to batch changes into regeneration runs. If we submit 1 CL per day for a week, no need to regenerate everything until the end of the week. If we automatically regenerate, it could lead to more churn than necessary.

[tbpg]

One way to avoid the churn caused by doc-templates would be to just... not look at doc-templates. So, we'd get rid of 2.iii in the logic above.

That would give library owners a good experience when they update the YAML. It would also allow us to batch changes to the templates.

[dandhlee]

The scope for now will be limited for 2.i (current behavior) and 2.ii. It will compare the YAML blob's time with the HTML blob's time to check and re-generate if possible.