specs: Move specs into docs/specs

[cason]

As part of the repository documentation and specs re-organization.

It would not be bad to keep a symlink specs -> docs/specs though.

[cason]

Are we still doing that? I don't find it really necessary.

One reason for considering this change is to render the full docs/ directory as the source for a potentially documentation website, as we have for CometBFT. In the Comet's documentation, we have not really nice workarounds to add the spec/ directory inside the documentation, which is derived from the docs/ directory. See cometbft/cometbft-docs#79 for more context.

[cason]

It would not be bad to keep a symlink specs -> docs/specs though.

While this is helpful for the transition period (namely, existing links don't break), this become a problem when the Markdown files use relative links (which is the ideal behavior).