docs: fix overview icons and add authentication page (#103)

## Summary
- Fix verify card icon: `check-badge` → `check-circle-solid` (exists in
registry)
- Fix cache card icon: `archive-box` → `folder-open` (exists in
registry)
- Add `docs/authentication.mdx` with provider token documentation
(GITHUB_TOKEN, GITLAB_TOKEN, GITEA_TOKEN)

**Required before next docs deploy** — sidebars.js on the `docs` branch
references the `authentication` doc ID.

## Test plan
- [ ] Merge to main triggers docs workflow rebuild
- [ ] Verify/cache cards show proper icons on overview page
- [ ] Authentication page renders at /authentication

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: fentas

docs/authentication.mdx

@@ -0,0 +1,72 @@
+---
+description: "Configure authentication tokens for private repositories and higher rate limits"
+---
+
+# Authentication
+
+`b` supports authentication tokens for accessing private repositories and increasing API rate limits. Tokens are read from environment variables.
+
+## Provider Tokens
+
+| Provider | Environment Variable | Header Format |
+| -------- | -------------------- | ------------- |
+| GitHub   | `GITHUB_TOKEN`       | `Authorization: Bearer <token>` |
+| GitLab   | `GITLAB_TOKEN`       | `PRIVATE-TOKEN: <token>` |
+| Gitea / Codeberg | `GITEA_TOKEN` | `Authorization: token <token>` |
+
+## Usage
+
+Export the appropriate variable before running `b`:
+
+```bash
+export GITHUB_TOKEN="ghp_..."
+b install owner/private-repo
+```
+
+### GitHub
+
+A GitHub token increases the API rate limit from 60 to 5,000 requests per hour and allows access to private repositories.
+
+```bash
+export GITHUB_TOKEN="ghp_..."
+```
+
+Without a token, `b` will show a warning when rate-limited:
+
+> GitHub API rate limited (set GITHUB_TOKEN for higher limits)
+
+### GitLab
+
+A GitLab token is required for private repositories on `gitlab.com` or self-hosted instances.
+
+```bash
+export GITLAB_TOKEN="glpat-..."
+```
+
+### Gitea / Codeberg
+
+A Gitea token is required for private repositories on Gitea or Forgejo instances like Codeberg.
+
+```bash
+export GITEA_TOKEN="..."
+```
+
+## CI/CD
+
+In CI/CD pipelines, use secrets or environment variables provided by your CI system:
+
+```yaml
+# GitHub Actions
+steps:
+  - name: Install tools
+    env:
+      GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+    run: b install
+
+# GitLab CI
+install_tools:
+  script:
+    - b install
+  variables:
+    GITLAB_TOKEN: $GITLAB_API_TOKEN
+```

docs/b/overview.mdx

@@ -93,7 +93,7 @@ b install github.com/org/infra:/manifests/** ./manifests
     href: '/b/subcommands/verify',
     label: 'b verify',
     customProps: {
-      icon: Icons['check-badge'],
+      icon: Icons['check-circle-solid'],
       description: 'Verify installed artifacts against b.lock checksums.'
     }
   },
@@ -102,7 +102,7 @@ b install github.com/org/infra:/manifests/** ./manifests
     href: '/b/subcommands/cache',
     label: 'b cache',
     customProps: {
-      icon: Icons['archive-box'],
+      icon: Icons['folder-open'],
       description: 'Manage the local git cache for env file syncing.'
     }
   },