fix(telegram): update docs and generator for private channels

[piksel]

The current docs regarding telegram channel names only applies to public channels. Addressing private channels is not possible using the described @channelname syntax.
This PR fixes the documentation regarding this, and makes the telegram generator display private channels correctly (instead of just calling them @).

Fixes #249

[codecov]

Codecov Report

Merging #250 (8a2f7bb) into main (468d552) will increase coverage by 0.11%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #250      +/-   ##
==========================================
+ Coverage   77.73%   77.85%   +0.11%     
==========================================
  Files          96       96              
  Lines        2866     2877      +11     
==========================================
+ Hits         2228     2240      +12     
+ Misses        468      466       -2     
- Partials      170      171       +1     

Impacted Files	Coverage Δ
pkg/services/telegram/telegram_generator.go	78.04% <100.00%> (+3.40%)	⬆️
pkg/services/telegram/telegram_json.go	94.44% <100.00%> (+5.55%)	⬆️

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[JosephKav]

I'd still recommend the forward message to 'JSON Dump Bot' method to get the chat ID. Doesn't require a PC/Docker compared to docker run --rm -it containrrr/shoutrrr generate telegram
Up to you though of course

[piksel]

@JosephKav Yeah, good point. My main concern is that it's a third party bot that is out of our control regarding what it's actually doing.
It's also not very user friendly.

Shoutrrr CLI works fine outside of docker and even runs on android. The only reason for recommending the docker way is that it's a single line to download, run and remove the CLI in one go (and it's always updated).
It's also possible to run the generator in the browser:
https://wasmcli.p1k.se/
But it's in alpha stage still... (it's also running a much older shoutrrr version here). Ideally it would be deployed with the docs sometime in the future...

[piksel]

@JosephKav I added another section, but replaced @JsonDumpBot with @UserInfoBot, which is made by the same author. It's slightly easier to use, but provides the same information.

[piksel]

Since neither @JsonDumpBot, nor @UserInfoBot could help getting the Chat ID for group chats, I wrote a bot specifically for this purpose. It can be reached at @shoutrrrbot. Updated the docs to point to it instead.