Accept CMS config in multiple formats

**- Do you want to request a *feature* or report a *bug*?**

neither, but, a suggestion for the docs.

**- What is the current behavior?**

hexo can disable the CMS non-intuitively.

**- If the current behavior is a bug, please provide the steps to reproduce.**

- run a hexo site
- put cms in /admin/
- use config.yml as your config file
- run hexo generate in /
- notice that config.yml is no longer present and CMS no longer loads in deployed site

**- What is the expected behavior?**

the docs would mention this :)

**- Please mention your node.js, and operating system version.**

(so really this is about the docs, not cms version)

Start with the reported Hexo flow: place the CMS in /admin/, use config.yml, and run hexo generate from the site root. Check how the generated site handles config.yml and the CMS, then update the relevant documentation to explain the configuration requirement and verify the documented steps match the reproduction.

Written by the indexing model from the issue text.