docs: Add etcupdate.rst

docs/chapters/subcommands/etcupdate.rst

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+=========
+etcupdate
+=========
+
+This command will update the contents of `/etc` inside a jail. It should be run after a jail upgrade
+
+First we need to bootstrap a release for `etcupdate` to use.
+
+.. code-block:: shell
+
+  ishmael ~ # bastille etcupdate bootstrap 14.1-RELEASE
+  bastille_bootstrap_archives: base -> src
+  /usr/local/bastille/cache/14.1-RELEASE/MANIFES        1046  B 1134 kBps    00s
+  /usr/local/bastille/cache/14.1-RELEASE/src.txz         205 MB 2711 kBps 01m18s
+  bastille_bootstrap_archives: src -> base
+  Building tarball, please wait...
+  Etcupdate bootstrap complete: 14.1-RELEASE
+
+Next we can use the `update` command to apply the update to the jail.
+
+.. code-block:: shell
+
+  ishmael ~ # bastille etcupdate ishmael update 14.1-RELEASE
+
+The output will show you which files were added, updated, changed, deleted, or have conflicts.
+To automatically resolve the conflicts, run the `resolve` command.
+
+.. code-block:: shell
+
+  ishmael ~ # bastille etcupdate ishmael resolve
+
+To show only the differences between the releases, use the `diff` command.
+
+.. code-block:: shell
+
+  ishmael ~ # bastille etcupdate ishmael diff 14.1-RELEASE
+
+.. code-block:: shell
+
+  ishmael ~ # bastille etcupdate help
+  Usage: bastille etcupdate [option(s)] [bootstrap|TARGET] [diff|resolve|update RELEASE]
+    Options:
+
+    -d | --dry-run          Show output, but do not apply.
+    -f | --force            Force a re-bootstrap of a RELEASE.
+    -x | --debug            Enable debug mode.