fix: preserve docs markdown line merging

[GermanJablo]

Summary

1. Preserve previous docs MDX import behavior by passing shouldMergeAdjacentLines = true when converting MDX to Lexical.
2. This pairs with refactor(richtext-lexical): remove vendored @lexical/markdown fork and upgrade to lexical 0.45 payload#17064, which removes Payload's vendored @lexical/markdown fork and uses upstream Lexical markdown behavior.

Test plan

1. Verified the importer file has no linter diagnostics.
2. Ran the roundtrip docs comparison from the local website checkout against the Payload docs.