Add reference/history material to Census Block Groups layers

[christopeak]

From Eric:
• Census Block Groups 2010, 2020: Are we including 2000? Also, I think it would be nice to include reference/history material, linked here. As with the blocks there may be other more specific documents for each vintage, but this is what I found.

[christopeak]

Census_Block_Groups_2000 is already on Portal.

[christopeak]

Also, I don't think we need to include histories of block group geographies in general as part of layer metadata. Do others disagree?