docs(lint): document 11 ported oxlint unicorn rules

[bartlomieju]

Adds documentation pages for the eleven oxlint "unicorn" correctness rules
being ported to deno_lint as native rules. Each page follows the existing
lint/rules/*.md format — recommended frontmatter tag, a short
description, prose explaining the rationale, and Invalid/Valid code
examples. The rule list is generated by walking lint/rules/, so no index
changes are needed.

Documented rules: no-await-in-promise-methods, no-empty-file,
no-invalid-fetch-options, no-invalid-remove-event-listener,
no-new-array, no-single-promise-in-promise-methods, no-thenable,
no-unnecessary-await, no-useless-fallback-in-spread,
no-useless-length-check, and prefer-set-size.

These pair with the rule implementations in deno_lint (PRs #1520, #1522–
#1531). All eleven rules are tagged recommended, mirroring the tag on the
rule implementations; that on-by-default choice is pending maintainer
sign-off on the deno_lint PRs, so the recommended frontmatter here should
be confirmed alongside those.