doc: add examples for every feature

For now, gotd have a lot badly documented features and helpers, like uploader package.

[paskal]

The project would benefit greatly from an examples folder with different scenarios like big file upload or examples per package to be available in the module documentation.

[ernado]

I've added documentation as current focus to roadmap.

Until then, feel free to ask questions in telegram groups, we have both English and Russian support channels.
Also there are upload in cli, so you can see https://github.com/gotd/cli/blob/ecd58c4e716de86cf8c37520989a8bb3391fe783/cmd/tg/upload.go#L162-L170 for reference for uploads.

[lilien1010]

We definitely need an example of：
user login by code with session storage in file
user reconnect with the previous session.
user send message to a channel.

[Vponed]

We definitely need an example of： user login by code with session storage in file user reconnect with the previous session. user send message to a channel.

Очень прошу пример с входом по коду и сохранением в сессию

[AlexanderMatveev]

Please add example of ChannelsGetParticipants method usage