Document new Data API interface additions

[gabizou]

Yesterday, I added ListData and MappedData along with their relevant common implementations for plugins to use. Note that due to the breaking changes of the common implementations, this had to go into bleeding.

They're very self explanatory as to what they do (they're more for "ease of use" interfaces for commonly used interfaces than anything else).

Note that the implementations have been of course updated for these changes, also in their respective bleeding branches.

[ST-DDT]

Target page: https://docs.spongepowered.org/stable/en/plugin/data/custom/datamanipulators.html

Can someone give me link to an example plugin with public source code that uses custom datamanipulators?

Then I will mention the potential parents along with the updated method calls.

Related: #640

[ST-DDT]

A quick search on ore and github didn't yield any results. Is there a plugin out there that actually uses custom datamanipulators? Is it because the docs are lacking?
Maybe https://github.com/SpongePowered/SpongeCommon/tree/stable-7/testplugins/src/main/java/org/spongepowered/test/myhomes could be used as an example/way to fix the docs.

[ST-DDT]

And now I know why: SpongePowered/SpongeAPI#1913

I'll wait with the documentation until the API is fixed.