docs: fix specs

[karmacoma-eth]

This covers different kinds of edits, from no-brainers to maybe controversial, so I separated them into individual commits for easier inspection.

• cee81d9 fix table of contents: the links were broken, which is why I started this PR in the first place, should be a no brainer
• 405adf5 use checksum address: similar, checksum addresses are just better
• 2972ac3 and 67b0fd9 are purely syntactic changes

Will call out the riskier changes inline.

Additionally, some open ended feedback (feel free to redirect the conversation):

• there are multiple references to "the Signer CRDT", but it's not defined in the spec
• I think it would make sense to define in one place what a valid fid is, and then say "field X must be a valid fid". There are references to "an fid is only valid if it is present in the Id registry" (what does this mean exactly?), something being "a known fid" (is it the same as being in the Id registry?), and some references to an fid being either >=0 or >0 (does this mean that a valid integer is ok even if it is not in the Id registry?)

[karmacoma-eth]

⚠️ calling this out: this is consistent with CastId defining fid as > 0. If the intention here is that fid can be explicitly 0, I think it should be stated otherwise it just looks inconsistent.

Also: does it mean that any fid is valid even if they are not in the Id registry?

[sds]

Thank you!