Add and manage guide documentation

[Yang-33]

I've found PhpDocumentor provides Hand written documentation.

• https://docs.phpdoc.org/guide/hand-written-docs/index.html

Current our docs is generated based on only php code, which is not helpful for beginners.
Let's add guide that is written manually, like other bot sdks.

The manual page is merged into docs on build by PhpDocumentor automatically, and preview is here:

As a first step, let's try to build page without docs dir. I've already tested in my local.
Next PR removes docs dir (#859), which is noisy on creating PR.

(part of #854)

[Yang-33]

Result

https://line.github.io/line-bot-sdk-php/docs/getting-started.html#getting-started