add qsymm to extensions

See merge request kwant/website!26

Add a page containing links to Kwant extensions

Nikola is not a fan of the nested structure for some reason.
Our number of posts is liable to remain small.

We don't check the host key and just upload the docs. The worst that
can happen is that an "attacker" gets our docs, and the Kwant website
does not get updated. We enforce the use of public key authentication
only, so an attacker cannot get any credentials, and the docs are
public information, so there is no risk.

Previously it was (incorrectly) a subsection of the 'conda' section.

Since Sympy 1.0 MPMath is an external dependency. Because Windows
users have to manually install all dependencies, we should also list
MPMath.