wopi: documentation

should probably provide a minimal working example for using copyparty with collabora and/or onlyoffice;

my extremely jank and dangerously unsafe testing setup was:

### copyparty server for collabora

```
rm -rf junk ; curl -LO http://192.168.123.1:3923/dist/copyparty-sfx.py && python3 copyparty-sfx.py -aed:wark -v junk::A,ed --wopi --wopi-url https://192.168.122.210:9980/ --wopi-crt no
```

### copyparty server for onlyoffice

```
rm -rf junk ; curl -LO http://192.168.123.1:3923/dist/copyparty-sfx.py && python3 copyparty-sfx.py -aed:wark -v junk::A,ed --wopi --wopi-url http://192.168.122.5:9980/ --wopi-crt no --ihead=\* --ohead=\*
```

### collabora itself (works)

```
podman run --rm -it --net=host -e server_name=192.168.122.210:9980 -e "extra_params=--o:ssl.enable=true --o:ssl.termination=true --o:storage.ssl.ssl_verification=false" collabora/code:latest
# then access https://192.168.122.210:9980/ from firefox to accept the selfsigned cert
```

### onlyoffice itself (failed attempt)

```
podman run --rm -it -p 9980:80 -p 9443:443 \
-v /home/ed/v/logs:/var/log/onlyoffice \
-v /home/ed/v/data:/var/www/onlyoffice/Data \
-v /home/ed/v/lib:/var/lib/onlyoffice \
-v /home/ed/v/etc:/etc/onlyoffice/documentserver \
-e WOPI_ENABLED=true -e USE_UNAUTHORIZED_STORAGE=true onlyoffice/documentserver
```

NOTE: the onlyoffice docs says setting up ip-address allow/deny-list is recommended to ensure only copyparty can connect, so should at least mention that; https://api.onlyoffice.com/docs/docs-api/more-information/faq/using-wopi/

NOTE: onlyoffice includes a "wopi proof key" in its requests which the server (copyparty) is expected to verify, to ensure that the document has not been tampered with and that it's coming from a trusted source, but is this necessary for our usecase? is the access-token sufficient, like for collabora?

this builds on #1495 and is related to #1574