Move pages under tldr, lint run.sh, update docs

tldr/linux/systemd-sysext

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+---
+syntax: markdown
+tags: [tldr, linux]
+source: https://github.com/tldr-pages/tldr.git
+---
+# systemd-sysext
+
+> Activate or deactivate system extension images.
+> More information: <https://www.freedesktop.org/software/systemd/man/systemd-sysext.html>.
+
+- List installed extension images:
+
+`systemd-sysext list`
+
+- Merge system extension images into `/usr/` and `/opt/`:
+
+`systemd-sysext merge`
+
+- Check the current merge status:
+
+`systemd-sysext status`
+
+- Unmerge all currently installed system extension images from `/usr/` and `/opt/`:
+
+`systemd-sysext unmerge`
+
+- Refresh system extension images (a combination of `unmerge` and `merge`):
+
+`systemd-sysext refresh`