docs: add comprehensive XPATH function examples

[jonathan-fulton]

Summary

Expand the xpath.fql example file with comprehensive examples of XPATH queries.

Context

As requested in #606, the documentation lacked practical examples showing how to use the XPATH function for common web scraping tasks. Users were unclear on how to query various types of metadata from HTML documents.

Changes

Enhanced examples/xpath.fql with examples demonstrating:

• Basic queries: Extracting charset, page title, and link hrefs
• Meta tag extraction: Getting description and other meta content by name
• Open Graph metadata: Extracting og:url, og:title, og:image (commonly needed for social sharing info)
• Twitter Card metadata: Extracting Twitter-specific meta tags
• Class-based selection: Using contains() for matching CSS classes (XPath 1.0 pattern)
• Position predicates: Selecting specific matches from result sets
• ID-based selection: Targeting elements by ID attribute
• Combined conditions: Using and operator in predicates

Example Output

// Get Open Graph metadata
LET ogUrl = XPATH(doc, "//meta[@property='og:url']/@content")
LET ogTitle = XPATH(doc, "//meta[@property='og:title']/@content")

Fixes #606

[ziflex]

Thank you!