Docs: Document installation of frontends

Co-authored-by: Roman Chvanikov <chvanikoff@pm.me>

docs/administration/CLI_tasks/frontend.md

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+# Managing frontends
+
+`mix pleroma.frontend install <frontend> [--ref <ref>] [--file <file>] [--build-url <build-url>] [--path <path>] [--build-dir <build-dir>]`
+
+Frontend can be installed either from local zip file, or automatically downloaded from the web.
+
+You can give all the options directly on the command like, but missing information will be filled out by looking at the data configured under `frontends.available` in the config files.
+
+Currently known `<frontend>` values are:
+- [admin-fe](https://git.pleroma.social/pleroma/admin-fe)
+- [kenoma](http://git.pleroma.social/lambadalambda/kenoma)
+- [pleroma-fe](http://git.pleroma.social/pleroma/pleroma-fe)
+- [fedi-fe](https://git.pleroma.social/pleroma/fedi-fe)
+- [soapbox-fe](https://gitlab.com/soapbox-pub/soapbox-fe)
+
+You can still install frontends that are not configured, see below.
+
+## Example installations for a known frontend
+
+For a frontend configured under the `available` key, it's enough to install it by name.
+
+```bash
+mix pleroma.frontend install pleroma
+```
+
+This will download the latest build for the the pre-configured `ref` and install it. It can then be configured as the one of the served frontends in the config file (see `primary` or `admin`).
+
+You can override any of the details. To install a pleroma build from a different url, you could do this:
+
+```bash
+mix pleroma.frontend install pleroma --ref 2huedition --build-url https://example.org/raymoo.zip
+```
+
+Similarly, you can also install from a local zip file.
+
+```bash
+mix pleroma.frontend install pleroma --ref mybuild --file ~/Downloads/doomfe.zip
+```
+
+The resulting frontend will always be installed into a folder of this template: `${instance_static}/frontends/${name}/${ref}`
+
+Careful: This folder will be completely replaced on installation
+
+## Example installation for an unknown frontend
+
+The installation process is the same, but you will have to give all the needed options on the commond line. For example:
+
+```bash
+mix pleroma.frontend install gensokyo --ref master --build-url https://gensokyo.2hu/builds/marisa.zip
+```