matrix: added main page and docker image page

wiki/docker-images/matrixdotorg_-_synapse.md

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+# matrixdotorg - synapse
+
+The official container and documentation was made by
+[matrixdotorg](https://hub.docker.com/matrixdotorg/synapse).
+
+## Volumes
+
+Set the following volumes in the `volumes:` section of the docker-compose file.
+
+| Volume-Name | Container mount | Description                     |
+| ----------- | --------------- | ------------------------------- |
+| `matrix`    | `/data`         | storage for all homeserver data |
+
+## Ports
+
+Set the following ports in the `ports:` section.
+
+| Container Port | Recommended outside port | Protocol | Description                       |
+| -------------- | ------------------------ | -------- | --------------------------------- |
+| `3478`         | `3478`                   | TCP      | TURN over TLS                     |
+| `5349`         | `5349`                   | TCP      | TURN over TLS                     |
+| `8008`         | `8008`                   | TCP      | standard port for matrix protocol |
+
+## Rebuild
+
+```shell
+#!/bin/sh
+docker-compose down
+docker pull matrixdotorg/synapse
+docker-compose up -d
+```
+
+## Docker-Compose.yml
+
+```yaml
+version: '3.3'
+
+services:
+  app:
+    image: matrixdotorg/synapse
+    restart: always
+    volumes:
+      - matrix:/data
+    ports:
+     - "3478:3478"
+     - "5349:5349"
+     - "8008:8008"
+volumes:
+    matrix:
+        driver: local
+```

wiki/linux/mkinitcpio.md

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+# mkinitcpio
+
+`mkinitcpio` is a script to create an initial ramdisk environment.
+
+## Troubleshooting
+
+### '/lib/modules/xxxxxxx' is not a valid kernel module directory
+
+When you encounter an error like the header of this section when running
+`mkinitcpio` try first to reinstall the `linux`, `linux-headers` and
+`filesystem` packages.
+If that does not fix it continue.
+Look into the `/lib/modules` directory.
+There should be a directory called `4.9.8-1-ARCH` or similar.
+Now run `mkinitcpio -k 4.9.8-1-ARCH -g /boot/image.img` and move the image
+to the correct location `/boot/initramfs-linux.img` in the boot directory (back
+up the old one).
+After that reboot and it should again work fine (Source:
+[Unix Stackexchange](https://unix.stackexchange.com/questions/395402/lib-modules-4-9-8-1-arch-is-not-a-valid-kernel-module-directory)).

wiki/matrix.md

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+# Matrix
+
+[Matrix](https://matrix.org) is an open communication protocol.
+
+## Server
+
+A (home-)server can be setup via docker with the
+[matrixdotorg image](./docker-images/matrixdotorg_-_synapse.md).
+
+After the setup the registration is disabled.
+To enable registration go to the `homeserver.yaml`
+and locate the following lines:
+
+```yaml
+## Registration ##
+
+# Enable registration for new users.
+enable_registration: False
+```
+
+Set `enable_registration` to `True` to enable sign up.
+
+It is recommended to use a reverse proxy (like [nginx](./nginx.md)) for the 8008
+port.