[fix] correct small errors in string, struct, and system function docs

[boluor]

Summary

Fixes 28 small documentation issues across the string, struct, and system function references. Each item below is independent.

string-functions

• ascii.md / auto-partition-name.md / char-length.md / cut-to-first-significant-subdomain.md / format-number.md / length.md / ngram-search.md / regexp-extract-all.md — frontmatter description was truncated mid-sentence; completed each.
• count_substrings.md — two list items were both numbered 7.; renumbered the second to 8..
• char.md — three result-table headers showed char('utf8', ...) while the queries used CHAR(...); headers now match the queries.
• is-uuid.md — the result-table header omitted the curly braces that were in the query; header now includes them.
• mask-last-n.md — the query used 'Helloṭṛ123' while the result header and output used 'Hello你好123'; query aligned with the displayed result.
• parse-url.md / protocol.md — a Chinese ## 相关命令 section (with Chinese body and full-width 。) was sitting in English docs; translated to ## See Also with English body.
• position.md / repeat.md / reverse.md — each file had a complete duplicate Description/Syntax/Parameters/Examples block appended at the end; the trailing duplicate was removed.
• quote.md — escape note contradicted itself (claimed both \\ → \\\\ and \\\\ → \\); removed the bad bullet. Also, quote(\"It's a test\") showed 'It's a test' (single quote unescaped) in the output; corrected to 'It\\'s a test'.
• parse-data-size.md — prose listed units up to ZB/YB but the table stops at EB; aligned prose with the table.
• hamming_distance.md / levenshtein.md — legacy frontmatter (title: … only, no language/description) updated to the standard JSON form used by sibling docs.
• regexp-count.md — typo is n the total count corrected; typos paratemer/usr corrected to parameter/user; the quadruple-backslash pattern [\\\\\\\\.:;] simplified to [\\\\.:;].
• regexp-extract-or-null.md — example query pattern had ([[]ower:]]+) while the error message referenced ([[:lower:]+); query updated to match the error.
• regexp.md — the greedy / non-greedy example invoked REGEXP_EXTRACT(...) instead of REGEXP(...); corrected. A stray ~ on its own line after ## Description (and a leading ~ in the frontmatter description) removed.
• soundex.md — examples were numbered 1-7 then jumped to 9; corrected to 8.
• url-decode.md / url-encode.md — result blocks were fenced as sql; corrected to text. url-encode.md also had a duplicate ## Required Parameters heading; removed.
• frontmatter descriptions that referenced function names without underscores (e.g. REGEXPREPLACEONE, MASKLASTN, PARSEURL, CHARLENGTH, ...) corrected to the canonical underscored form (REGEXP_REPLACE_ONE, MASK_LAST_N, PARSE_URL, CHAR_LENGTH, ...).

system-functions

• database.md — SELECT database(), schema() showed both result-table headers as database(); second header corrected to schema().

struct-functions

• named-struct.md / struct.md / struct-element.md — frontmatter language: \"en-US\" → \"en\".

Test plan

• CI doc build passes
• Spot-check the affected pages render correctly (completed descriptions, no duplicate trailing blocks, corrected result-table headers, no stray punctuation, language metadata in struct docs)