docs(examples): add READMEs for p/nt packages

[davd-gzl]

Adds documentation for the p/nt packages, following a consistent format: summary, usage example, API reference, and notes. Three packages that had none get new READMEs: markdown/foreign, markdown/sanitize, and ownable/exts/authorizable.

commondao is intentionally left out to keep the scope on the simpler packages.

Continues the earlier draft #4817.

[Gno2D2]

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[notJoon]

Left a few nits, but looks good to me 👍