refactor(swap): rename imported .md files to md- prefix for swap mechanism

Restore the canary file naming convention that swap_sources.py expects:
the imported MyST pages now live as docs/<dir>/md-<name>.md alongside
the existing docs/<dir>/<name>.rst, so swap_sources.py --swap can rename
them into place at build time.

- 254 .md files renamed (every page with a matching .rst counterpart)
- 2 MyST-only pages left at their final names (no .rst exists, no swap
  needed): docs/copyright.md, docs/automation/terraform/terraformvyos.md

All 114 stems listed in docs/_swap.txt now have a corresponding
md-<name>.md source file ready to swap in.

🤖 Generated by [robots](https://vyos.io)

1 files changed, 105 insertions, 0 deletions

diff --git a/docs/installation/md-update.md b/docs/installation/md-update.md
new file mode 100644
index 00000000..e8b5f912
--- /dev/null
+++ b/docs/installation/md-update.md
@@ -0,0 +1,105 @@
+---
+lastproofread: '2026-01-26'
+---
+
+(update_vyos)=
+
+# Update VyOS
+
+New system images can be added using the {opcmd}`add system image` command.
+This command extracts the image and prompts you to use the current system
+configuration and SSH security keys, allowing the new image to boot with your
+current configuration.
+
+:::{note}
+Only LTS releases are PGP-signed.
+:::
+
+```{opcmd} add system image \<url | path\> | [latest] [vrf name] [username user [password pass]]
+
+Use this command to install a new system image. You can retrieve the
+image from the web (``http://``, ``https://``) or from your local system.
+For example: /tmp/vyos-1.2.3-amd64.iso.
+
+   The ``add system image`` command also supports installing new VyOS versions
+   through an optional VRF. If the URL requires authentication, you can specify
+   an optional username and password on the command line, which will be passed
+   as "Basic-Auth" to the server.
+```
+
+If there isn't enough free disk space, the installation will be canceled.
+To delete images, use the {opcmd}`delete system image` command.
+
+
+VyOS associates configuration with each image, and each image has its own
+unique configuration copy. This differs from traditional network routers where
+the configuration is shared across all images.
+
+
+:::{note}
+If you have personal files such as scripts that you want to preserve
+during the upgrade, store them in `/config` since this directory is always
+copied to newly installed images.
+:::
+
+
+You can access files from a previous installation and copy them to your
+current image if they were stored in the `/config` directory. Use the
+{opcmd}`copy` command to do this. For example, to copy `/config/config.boot`
+from the VyOS `1.2.1` image, run:
+
+```none
+copy file 1.2.1://config/config.boot to /tmp/config.boot.1.2.1
+```
+
+
+#### Example
+
+```none
+vyos@vyos:~$ add system image https://s3.amazonaws.com/s3-us.vyos.io/rolling/current/vyos-1.4-rolling-202201120317-amd64.iso
+Trying to fetch ISO file from https://s3.amazonaws.com/s3-us.vyos.io/rolling/current/vyos-1.4-rolling-202201120317-amd64.iso
+  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
+                                 Dload  Upload   Total   Spent    Left  Speed
+100  338M  100  338M    0     0  3837k      0  0:01:30  0:01:30 --:--:-- 3929k
+ISO download succeeded.
+Checking for digital signature file...
+  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
+                                 Dload  Upload   Total   Spent    Left  Speed
+  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
+curl: (22) The requested URL returned error: 404 Not Found
+
+Unable to fetch digital signature file.
+Do you want to continue without signature check? (yes/no) [yes]
+Checking MD5 checksums of files on the ISO image...OK.
+Done!
+
+What would you like to name this image? [vyos-1.3-rolling-201912201452]:
+
+OK.  This image will be named: vyos-1.3-rolling-201912201452
+```
+
+You can use `latest` option. It loads the latest available Rolling release.
+
+```none
+vyos@vyos:~$ add system image latest
+```
+
+:::{note}
+To use the `latest` option, "system update-check url" must be
+configured appropriately for your installed release.
+
+For updates to the Rolling Release for AMD64, the following URL may be
+used:
+
+<https://raw.githubusercontent.com/vyos/vyos-nightly-build/refs/heads/current/version.json>
+:::
+
+:::{hint}
+You can access the latest Rolling Release for AMD64 from a web
+browser at:
+
+<https://vyos.net/get/nightly-builds/>
+:::
+
+After rebooting, verify the version you're running using the
+{opcmd}`show version` command.