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/installation.md

@@ -0,0 +1,37 @@
+---
+sidebar_position: 2
+---
+
+# Installation
+
+gt is a single binary for Linux, macOS, Windows and FreeBSD.
+
+## Install script
+
+The install script downloads gt from [dl.gitea.com](https://dl.gitea.com/gitea-cli/),
+verifies its checksum and installs it.
+
+```bash
+# Linux, macOS, FreeBSD
+curl -fsSL https://gitea.com/gitea/cli/raw/branch/main/scripts/install.sh | sh
+```
+
+```powershell
+# Windows
+irm https://gitea.com/gitea/cli/raw/branch/main/scripts/install.ps1 | iex
+```
+
+Set `GITEA_CLI_VERSION` to install a specific version, or `nightly` for the latest build
+of `main`.
+
+## Other ways
+
+- **Binaries:** download them from [dl.gitea.com/gitea-cli](https://dl.gitea.com/gitea-cli/).
+- **Docker:** the [`gitea/gitea-cli`](https://hub.docker.com/r/gitea/gitea-cli) image runs
+  `gt` as its entry point.
+- **Go:** `go install gitea.dev/cli/cmd/gt@latest`
+
+## Shell completion and man pages
+
+`gt completion <shell>` prints the completion script for bash, zsh, fish or PowerShell;
+`gt completion <shell> --help` shows how to load it. `gt man` prints the manual.