docs: restructure documentation and improve newcomer experience (#2573)

* feat: add --write-output flag to helmfile fetch for air-gapped environments

Add --write-output flag to helmfile fetch that outputs a modified
helmfile.yaml with chart references updated to point to downloaded
local chart paths. Combined with --output-dir, this enables preparing
all charts for deployment in air-gapped environments.

Usage:
  helmfile fetch --output-dir ./charts --write-output > helmfile-airgapped.yaml

Fixes #2571

Signed-off-by: yxxhero <yxxhero@users.noreply.github.com>
Signed-off-by: yxxhero <aiopsclub@163.com>

* docs: restructure documentation and improve newcomer experience

Split the monolithic index.md (1990 lines) into focused topic pages,
update mkdocs.yml navigation, and add missing documentation for
undocumented code features.

Structure changes:
- Extract configuration.md (helmfile.yaml reference)
- Extract cli.md (CLI commands and flags)
- Extract templating.md (template syntax and env vars)
- Extract environments.md (environment configuration)
- Extract releases.md (DAG, needs, selectors)
- Extract hooks.md (lifecycle hooks)
- Extract integrations.md (ArgoCD, Azure ACR, OCI)
- Slim index.md to ~270 line landing page with step-by-step tutorial

Newcomer improvements:
- Add 5-step Getting Started tutorial with explanations
- Reorganize nav: Getting Started now shows core learning path
  (Writing Helmfile → Values → Environments → Releases)
- Add Quick Reference table to configuration.md
- Simplify writing-helmfile.md title

Code-vs-docs gap fixes:
- Document 23 undocumented release fields (valuesTemplate,
  setTemplate, forceNamespace, adopt, trackMode, etc.)
- Document 6 undocumented helmDefaults fields (enableDNS,
  forceConflicts, skipRefresh, takeOwnership, etc.)
- Document print-env command and missing CLI flags
- Document kubectlApply hook field
- Document environment defaults field and merge order
- Document kubedogQPS/kubedogBurst advanced settings
- Document template partials (_*.tpl) auto-loading

Cleanup:
- Fix Docker image version from v0.156.0 to v1.1.0
- Fix heading nesting in advanced-features.md
- Update experimental-features.md with current features
- Fix broken cross-references and anchor links

Signed-off-by: yxxhero <aiopsclub@163.com>

* Revert changes to pkg/app from docs/restructure-and-improve branch

Signed-off-by: yxxhero <aiopsclub@163.com>

* docs: add create subcommand to README and CLI reference

Signed-off-by: yxxhero <aiopsclub@163.com>

---------

Signed-off-by: yxxhero <yxxhero@users.noreply.github.com>
Signed-off-by: yxxhero <aiopsclub@163.com>

docs/builtin-objects.md

@@ -1,4 +1,6 @@
-# helmfile template built-in objects
+# Built-in Objects
+
+## helmfile template built-in objects
 
 - `Environment`: The information about the environment. This is set by the
   `--environment` flag. It has several objects inside of it: