Related to 69dce03 (PR #2019).

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/2022

Ref: https://github.com/matrix-org/matrix-appservice-discord/pull/826

```
docker pull ghcr.io/matrix-org/matrix-appservice-discord:v3.0.0
v3.0.0: Pulling from matrix-org/matrix-appservice-discord
751ef25978b2: Pull complete 
16af4ec8b188: Pull complete 
8c8f56f7dc53: Pull complete 
afa016f2f989: Pull complete 
683e2bbbda4e: Pull complete 
fb056adbb1d6: Pull complete 
716f59a72dc7: Pull complete 
d52c94fc1da3: Pull complete 
b7b9cd5ddbeb: Pull complete 
Digest: sha256:1009697517bfe07a0d2192cf3b982bc2dbe40829cedc82c100aef61f8a43e3de
Status: Downloaded newer image for ghcr.io/matrix-org/matrix-appservice-discord:v3.0.0
ghcr.io/matrix-org/matrix-appservice-discord:v3.0.0
```

There is no docker tag in UI, but it exists:

```
docker pull registry.gitlab.com/signald/signald:0.21.0
0.21.0: Pulling from signald/signald
2f42a0d7a7b7: Pull complete 
81c2fb1b6074: Pull complete 
7e8b9a51d6b6: Pull complete 
f6ed8fd77301: Pull complete 
64e6fa036bdc: Pull complete 
5e98b5369603: Pull complete 
f2a9b80dd9fc: Pull complete 
Digest: sha256:201cf93efba689aa0319d2a480deea8ffb7dcdfbda31ea45932fccf4c7626ab9
Status: Downloaded newer image for registry.gitlab.com/signald/signald:0.21.0
registry.gitlab.com/signald/signald:0.21.0
```

* Fix bug that prevented user with external nginx from launch

The backslash was missing and prevented users from starting the bot

* Add necessary config for ext nginx to docs

* Add automatic config for ext nginx, adjust docs

* Remove unneeded and possibly puzzeling documentation

Move the `matrix_dendrite_process_extra_arguments` line up so it doesn't get lost, and add a trailing backslash

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/1996

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1992

Regression since cf6e38a586

Related to:

- https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1992
- https://github.com/kelaresg/go-skype-bridge/pull/17

Looks like we've skipped a bunch of 0.8.x versions (up to 0.8.9)
and are jumping straight to 0.9.0.

This is untested. Judging by Dendrite's changelog, it shouldn't cause
any breakage though: https://github.com/matrix-org/dendrite/blob/v0.9.0/CHANGES.md

This is untested.