Docs for #161 (Github issue) and #168 (Github Pull Request).

Related to #108 (Github Pull Request).

As suggested in #63 (Github issue), splitting the
playbook's logic into multiple roles will be beneficial for
maintainability.

This patch realizes this split. Still, some components
affect others, so the roles are not really independent of one
another. For example:
- disabling mxisd (`matrix_mxisd_enabled: false`), causes Synapse
and riot-web to reconfigure themselves with other (public)
Identity servers.

- enabling matrix-corporal (`matrix_corporal_enabled: true`) affects
how reverse-proxying (by `matrix-nginx-proxy`) is done, in order to
put matrix-corporal's gateway server in front of Synapse

We may be able to move away from such dependencies in the future,
at the expense of a more complicated manual configuration, but
it's probably not worth sacrificing the convenience we have now.

As part of this work, the way we do "start components" has been
redone now to use a loop, as suggested in #65 (Github issue).
This should make restarting faster and more reliable.

This change is provoked by a few different things:

- #54 (Github Pull Request), which rightfully says that we need a
way to support ALL mxisd configuration options easily

- the upcoming mxisd 1.3.0 release, which drops support for
property-style configuration (dot-notation), forcing us to
redo the way we generate the configuration file

With this, mxisd is much more easily configurable now
and much more easily maintaneable by us in the future
(no need to introduce additional playbook variables and logic).