Add documentation page for `FallbackChatGenerator`

[sjrl]

FallbackChatGenerator is a wrapper that tries multiple chat generators sequentially in case one earlier in the list fails.

It was introduced in #9859.

We should create a documentation page for it in https://docs.haystack.deepset.ai/, similarly to the other Chat Generators

[dfokina]

https://docs.haystack.deepset.ai/v2.19-unstable/docs/fallbackchatgenerator