Add isavasyopanishad (madhyandina)

[epicfaace]

Add isavasyopanishad (madhyandina recension). This comes as an export from Ambuda's proofing for this text -- https://ambuda.org/proofing/isavasyopanishad-text-only/

@akprasad regarding folder structure -- I just made a folder called "other_texts" where we can begin storing the texts we create using Proofing. Eventually we could then rename this repository to texts and then potentially reorganize the folders further?

[akprasad]

Sorry for the delay here -- I was thinking through where I want these texts, but I still don't have a great answer. I'm a little more cavalier with code, but I want to think through how we store this data and represent its lineage, not only within the document but also within the repo.

In particular, I want to think through the case where we have a text in the repo that someone requests we remove. Do we just strip that text from the repo and rewrite history? Or perhaps we could have multiple repos based on the text source or license guarantees.

I'm likely overthinking this, but I'd rather overthink our approach here and get the approach right than pick something reasonable and have to shuffle texts around. I'm curious what you think as well.

[epicfaace]

@akprasad I think that if someone requests we remove a text, and we decide to do so, it should be fine to rewrite history. I'd imagine this may not be a routine occurrence -- it would occur rarely enough that rewriting history should be OK. it's also worth reading GitHub's DMCA takedown policy, particularly the section around forks.

I think multiple repos would get logistically pretty complicated to handle, but we might want to revisit that question once we get a large enough volume of content contributed to Ambuda.