Move 'excerpts' around, only include if file exists

[jabrah]

Fixes #49

Prerequisite to automating the generation of Wax pages, search index, and image derivatives

• Instead of moving to _data, where only limited filetypes can be read, moving them to _keywords/excerpts/ seems to work OK
• Updated logic to check for excerpt page before including looks kind of weird because of how various filters work (or don't work) with variables

Per automation plans:

In the future, we can still remove _keywords/*.md files and keep _keywords/excerpts/ pages. We can then regenerate the Wax pages on demand and the excerpts should be included as expected (tested locally)

[egpalazzolo]

I really like this! That had been bothering me about the build a lot, not to mention including the transcription and translations in "data" seems like a better fit.