Merge extending Bard docs into Bard fieldtype docs

[joshuablum]

With our docs restructure both the Bard Fieldtype and Extending Bard docs share the same slug of bard. Pre restructure it was fieldtypes/bard and extending/bard but now it both lives in fieldtypes/{page}. So even though the Extending Bard page technically is still there you always see the Bard Fieldtype docs. Changed slug to extending-bard so it actually works again.

Closes #1812.

[duncanmcclean]

We should probably combine the pages instead.