1.0: Use `{% include %}` directive for footer content

# Problem

If we need to modify footer content in each page, we should also modify ~170 page files.
It seems better to split it into a separate file.

I've tried to use `{% include %}` with https://github.com/fluent/fluentd-docs-gitbook/pull/322.
It works in local environment, but when it was reverted even though it was merged once.
(I've heard that it was not worked as expected :disappointed: )

# Expected

To keep mainteinanability, use `{% include %}` or something good alternative for docs.fluentd.org.

# Additional Information


I've tested with the following version.

```
% gitbook --version
CLI version: 2.3.2
GitBook version: 3.2.3
```