feat: rework docs api

[tjjfvi]

Before, docs were a builtin part of the metadata, and added like so:

const $bar = $.withMetadata(
  $.docs("lalala"),
  $foo,
)

This generally made the metadata harder to consume.

Now, the docs are not specialized at all. They're simply another codec factory:

const $bar = $.documented(
  "lalala",
  $foo,
)

This codec factory is designed such that the resultant codec has both the metadata of $.documented and the metadata corresponding to the original $foo. This means that CodecVisitors will ignore $.documented by default:

new $.CodecVisitor<string>()
  .add($foo, () => "$foo")
  .visit($bar) // "$foo"

However, you can explicitly opt-in to using $.documented:

new $.CodecVisitor<string>()
  .add($foo, () => "$foo")
  .add($.documented, (_codec, docs) => docs)
  .visit($bar) // "lalala"

This $.documented can be written entirely in user-land (in contrast to the last approach), but we include it here for standardization. However, users can write similar codec factories if they'd like to attach other miscellaneous metadata.

Resolves #136