docs(site): add SEO description frontmatter to all 41 doc pages

None of the 41 documentation pages had a `description:` frontmatter, so
Google had nothing to use for SERP snippets. Add a unique, query-matched
description to every page — each leads with the real search term users
type ("Selenium retry", "Selenium parallel execution", "Selenium download
file", "Selenium accessibility testing") and states the outcome.

This is the lowest-effort, highest-leverage discoverability win: it doubles
as the "elevator pitch on every page" for search results without adding a
banner to the rendered docs.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: shossain786