docs: refactor docs/spec to use mdbook

docs/spec/.gitignore

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+book

docs/spec/README.md

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+# EigenDA Spec
+
+Built using [mdBook](https://rust-lang.github.io/mdBook/index.html).
+
+Meant to contain technical overviews, spec, and low-level implementation details related to EigenDA, as opposed to the [docs](https://docs.eigenda.xyz/) site which is meant to contain more introductory and high-level material.
+
+## Images
+
+`./src/assets` is a symlink to `docs/assets` in order for MdBook to be able to access the images and other assets in the `docs` directory, given that it seems to only have access to the `src` directory.
+
+Still not quite sure what's the best directory structure. Perhaps we could simply move the assets directory entirely under book/src, but then other READMEs throughout the repo that need access to those assets would need to be updated.

docs/spec/book.toml

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+[book]
+authors = ["Samuel Laferriere"]
+language = "en"
+multilingual = false
+src = "src"
+title = "EigenDA Spec"
+
+[output.html]
+mathjax-support = true