Break down the plugin development page

[kiyoto]

Currently, docs.fluentd.org/v0.12/articles/plugin-development has too much content. It should be broken down to separate pages per plugin (output, input, filter, parser, formatter) with more detailed guidelines on each page.

In particular, each page should have the following:

1. When should I develop my custom input/output/filter/parser/formatter plugin?
2. Have you already looked at ?
3. How to write a custom plugin.
4. How to release a custom plugin.

Then, docs.fluentd.org/v0.12/articles/plugin-development should really become just an overview page with links to the individual pages.