Simplify the runner docs and fix their outdated facts (#600)

The runner pages had drifted from the runner. They claimed Podman is unsupported, a registration retry limit of 10, a `container.env` proxy override that has no effect, and they used the deprecated `generate-config` command, while the actual instructions were buried under implementation details.

The develop and 5.x pages are rewritten from the current runner sources (`main` is `v5.2.0`). They are shorter and aimed at operators, with a new page covering what jobs get. The reference generator now documents the `config` command, and the quickstart pages in every language use the `gitea-runner` binary name.

Reviewed-on: https://gitea.com/gitea/docs/pulls/600
Reviewed-by: Lunny Xiao <[email protected]>
Co-authored-by: silverwind <[email protected]>

runner-docs_versioned_docs/version-3/reference/config-example.md

@@ -7,10 +7,8 @@ description: The commented example configuration of the runner, generated from t
 
 {/* Generated by update_runner_docs.sh from the gitea/runner sources, do not edit. */}
 
-This is the output of `gitea-runner generate-config`. It is safe to use unmodified,
-and it is the authoritative list of every option the runner understands. See
-[Configuration](../configuration.md) for what the options mean and how the file is
-loaded.
+Every option the runner understands, with its default, as printed by `gitea-runner config generate`.
+See [Configuration](../configuration.md) for how to write and load a config file.
 
 ```yaml
 # Every option with its default value, all commented out. Read this file, do not copy it.