add a missing command to set the special Gluon MAC address of the private WLAN interface to the documentation.

With all the packages being built 10GB are not enough anymore.

Closes: #1436

Let sites leave these texts empty by default, just add a comment where to
find the default labels.

Explain the magic of gluon.mk. The feature flag documention is moved into
this new page.

Adds support for Alfa AP121F USB powered travel router (looks like an USB ethernet adapter).
There is no factory image, but only a sysupgrade file for this router which can be installed using the inbuilt UBoot web interface.
Configure your PC with IP address 192.168.1.2 and connect the router using its RJ45 port. Hold the reset button while powering the device until all LEDs have flashed 3 times simultaneously. You can now access the fallback web interface using a web browser at http://192.168.1.1 and flash the sysupgrade file.

gluon-config-mode-contact-info: provide enhancements for german, english and french translation to comply with DSGVO (#1394)

* do not allow to obligatorily require contact information
* add remark that the data is provided voluntarily
* mention how to delete the data
* be very clear about the fact that the data being entered is public and
  can be downloaded and processed by anyone.

based on package documentation, authored by T_X 
https://github.com/freifunk-gluon/gluon/blob/84a6f65f02d7e36a073ba2839712f0c0bb1dda10/package/gluon-ebtables-limit-arp/Makefile#L18-L39

fixes #1383

In multidomain setups, VXLAN is enabled by default, but can be disabled in
domain configs using the mesh/vxlan option. In single domain setups, the
mesh/vxlan option is mandatory.

The UCI option for legacy mode is removed.

Fixes #1364