Add documentation for gt, the Gitea CLI (#603)

Adds a CLI product for gt (https://gitea.com/gitea/cli), set up like the runner, with short guides for installation, logging in, CI and scripting. Each command's details stay in gt <command> --help. Until the first release, the docs are served at /cli/; make cut-version PRODUCT=cli VERSION=0.1 adds the release series.

Closes https://gitea.com/gitea/cli/issues/16

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Co-authored-by: silverwind <[email protected]>
Reviewed-on: https://gitea.com/gitea/docs/pulls/603
Reviewed-by: Lunny Xiao <[email protected]>
Reviewed-by: silverwind <[email protected]>

cli-docs/index.md

@@ -0,0 +1,38 @@
+---
+sidebar_position: 1
+slug: /
+---
+
+# Gitea CLI
+
+[gt](https://gitea.com/gitea/cli) is the official command line client for Gitea. It works
+with repositories, issues, pull requests, releases, Actions and the other resources of a
+Gitea instance, from a terminal or a script.
+
+:::info Development version
+These pages describe builds from the `main` branch, published as `nightly` binaries and images.
+:::
+
+gt needs Gitea 28 or later, and Git for the commands that clone, check out or push.
+
+## Getting started
+
+1. [Install gt](installation.md).
+2. [Log in](authentication.md):
+
+   ```bash
+   gt auth login
+   ```
+
+3. Run commands in a clone of a repository on that instance, or select one with `--repo`:
+
+   ```bash
+   gt issue list --label bug
+   gt pr checkout 13
+   gt issue view 42 --repo gitea.com/gitea/cli
+   ```
+
+`gt doctor` checks your setup and suggests a fix for each problem it finds.
+
+`gt <command> --help` lists the flags of a command, and `gt help environment` the global
+flags and environment variables.