This commit adds copyright attributions in SPDX to the files for matrix-conduwuit, following REUSE's specification.

Signed-off-by: Suguru Hirahara <acioustick@noreply.codeberg.org>

This:

- brings consistency - no more mixing `_name_prefix` and `_registry_prefix`
- adds extensibility - a future patch will allow reconfiguring all registry prefixes for all roles in the playbook

We still have `_docker_` vs `_container_` inconsistencies.
These may be worked on later.

https://github.com/Virkkunen and I have been working on the same feature
simultaneously.

This patch adds my additional independent work on top of his changes.

Notable changes here compared to Virkkunen's original work:

- renaming config-related variables (`matrix_conduwuit_*` ->
  `matrix_conduwuit_config_*`). This is consistent with other roles (and
  better), but deviates from how the Conduit role was implemented.

- using a full configuration sample from https://conduwuit.puppyirl.gay/configuration.html
  instead of using the old Conduit config file template

- introducing configurability for more Conduwuit config settings, to support easy registration, etc.

- removing configuration settings that Conduwuit does not support
  anymore (e.g. `max_concurrent_requests`)

- cross-linking docs pages for Conduit and Conduwuit

- a Conduwuit docs page which is much improved compared to the old
  Conduit one

- REAMDE updates