Update docs to explain layout page variable

[yoshinorin]

hexojs/hexo#3355

[stevenjoezhang]

Docs: https://github.com/hexojs/site/blob/master/source/docs/templates.md#layouts

[sukrosono]

Hi 2021,
I add more expectation to this,
I expect we have a comprehensive guide or how-to on the doc site.

Here is the problem for me as newcomers
I want to build a custom layout, it might differ from Jekyll.
I want to design several layouts for my site, ie:
landing page layout and specify layout which later can be included as a partial template on other layouts.
to simplify I just need a reusable template.

Please let me know if there is already an existing feature to do that, but it will leave confusion later,
I expect i will use cli every time create a new post
hexo new yourlayout title right?