Document how to setup Discord as upstream OAuth 2.0 provider, add missing token_endpoint_auth_method field in GitHub sample config

[ChurchOfTheSubgenius]

The sample config for Github is missing the token_endpoint_auth_method field. Mas-cli won't sync config unless the field exists, and errors with:

Error: missing field `token_endpoint_auth_method` for key "default.upstream_oauth2.providers.0" in config.yaml YAML file

Adding token_endpoint_auth_method: "client_secret_post" fixes it.

[CLAassistant]

All committers have signed the CLA.

[ChurchOfTheSubgenius]

I've added sample upstream config for Discord, and a brand logo svg. Probably should have been a separate PR...

[sandhose]

I'd appreciate this to be either in a separate PR, or if you renamed the PR accordingly (as this is what ends up in the changelog)

[ChurchOfTheSubgenius]

I've renamed the PR. Will try to keep it cleaner next time..

[sandhose]

Could you add this in the comment here:

matrix-authentication-service/crates/config/src/sections/upstream_oauth2.rs

Lines 424 to 436 in bcaa783

	/// A brand identifier used to customise the UI, e.g. `apple`, `google`,
	/// `github`, etc.
	///
	/// Values supported by the default template are:
	///
	/// - `apple`
	/// - `google`
	/// - `facebook`
	/// - `github`
	/// - `gitlab`
	/// - `twitter`
	#[serde(skip_serializing_if = "Option::is_none")]
	pub brand_name: Option<String>,

and then regenerate the config schema with sh misc/update.sh

[ChurchOfTheSubgenius]

Done.