[MonologBundle] Document the available formatters

[stollr]

In the monolog documentation I cannot find any information about which built-in formatters are available. And the explanation how to use them is hidden in the article How to Define a Custom Logging Formatter linked in the Learn more paragraph of the main article.

I'd like to make two suggestions:

• Add a list of formatters, which are provided by the monolog bundle with a short description. If this is not desired, a link should be added so that users do not have to search in the bundle config files which formatters are available.
• The explanation about how to use the built-in formatters should be moved the main article, as the current place (custom logging formatter) is very unintuitive.

As an alternative one could create a dedicated page for formatters, like it was done for handlers.

if you agree with this suggestions, I'd be happy to create a PR. What do you think?