Integrate yarn content move into PR workflows for title-changing PRs

[yarusome]

Currently, moving contents around requires using yarn content move <from-slug> <to-slug> [locale], and there are mainly two types of PRs need to do this:

1. Changing the paths of pages except for anything after the last slashes;
2. Editing the titles of pages.

However, it's unnecessary to require contributors to run the command manually for type 2 PRs since all the command parameters, i.e. from-slug, <to-slug>, and locale, can be easily deduced from the front matter and the file paths. Thus it's more ergonomic to incorporate this command into workflows for PRs that edit titles but don't really^[1] move pages around.

A side gain is that contributors can even edit titles through the GitHub UI without having to install all the toolchains.

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[1] The pr-test workflow already can tell which files are renamed.

[Josh-Cena]

Editing page titles doesn't necessarily change the slug, and I would prefer it to not by default; fewer redirects is better for maintenance.