More documentation around what mutable paths exist

**Path to page with conent (_do not_ include domain name):**
content/en/os/1.13.x/concepts/restricted-filesystem/index.markdown

**What content is incorrect?**
I expected there to be some discussion about what is specifically stored outside of the immutable root filesystem. There is state such as logs and container images that would be useful to call out so a user can find them if they need them.

**What should the content say or communicate?**

I would like to see specific paths/locations for commonly asked for files. Logs comes to mind first, but it would be useful to call out `/local` and what exists in that path and when to expect it to be there or not (for instance, are these files always persisted or cleaned up during upgrades, etc)

Start with content/en/os/1.13.x/concepts/restricted-filesystem/index.markdown and review the existing explanation of the immutable root filesystem. Document specific locations for logs, container images, and /local, including when they exist, whether they persist, and what upgrades do to them; the page is done when it answers those user questions clearly.

Written by the indexing model from the issue text.