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/ci.md

@@ -0,0 +1,40 @@
+---
+sidebar_position: 4
+---
+
+# Using gt in CI
+
+Give gt a token and an instance through the environment, and turn prompts off:
+
+| Variable | Description |
+| --- | --- |
+| `GITEA_TOKEN` | Access token; gt never stores it |
+| `GITEA_CLI_HOST` or `GITEA_INSTANCE_URL` | URL of the instance |
+| `GITEA_CLI_REPO` | Repository, like `--repo` |
+
+`--no-input` makes a command fail instead of prompting, and `--yes` confirms changes in
+advance.
+
+## Gitea Actions
+
+Gitea does not allow secret names that start with `GITEA_`, so store the token as, for
+example, `GT_TOKEN`:
+
+```yaml
+on:
+  push:
+    branches: [main]
+
+jobs:
+  staging:
+    runs-on: ubuntu-latest
+    env:
+      GITEA_TOKEN: ${{ secrets.GT_TOKEN }}
+      GITEA_CLI_HOST: ${{ gitea.server_url }}
+      GITEA_CLI_REPO: ${{ gitea.repository }}
+    steps:
+      - run: curl -fsSL https://gitea.com/gitea/cli/raw/branch/main/scripts/install.sh | sh
+      - run: gt workflow run deploy.yml --ref main --field env=staging --no-input
+```
+
+`gt run watch` and `gt pr checks --watch` wait for runs and checks and fail when one fails.