docs(table-design): clarity pass on Index Overview scenario prose

[dataroaring]

What

A focused readability pass on the index mechanism descriptions in Index Overview > "How Each Index Works" (Scenarios 1-2): the prefix / inverted / ZoneMap / BloomFilter / NGram explanations.

• Split long run-on sentences into short declarative ones.
• Active voice with Doris as the subject (e.g. "Doris builds an inverted table…" instead of passive constructions).
• Remove the marketing word "more powerful" ("replaced by the more powerful inverted index" → "replaced by the inverted index").

No technical meaning changed. Reference tables (comparison, operators) and the capability lists in Scenarios 3-4 are left as-is, since terse reference content is appropriate there.

Scope

• Touches only the scenario prose. It does not change the "Start Here" section, which is in PR docs(table-design): make Index Overview default-first #3941; the two PRs edit different regions of the same file.
• EN + 中文; applies to both docs/ and versioned_docs/version-4.x.

Why

This content reads as dense, translated-from-source prose (60+ word single sentences, marketing adjective). The rest of the page was made task-first in #3941; this brings the explanation prose to the same clarity bar.