docs: add ZenRows provider and tool integration docs (#31742)

**Description:** Adds documentation for ZenRows integration with
LangChain, including provider overview and detailed tool documentation.
ZenRows is an enterprise-grade web scraping solution that enables
LangChain agents to extract web content at scale with advanced features
like JavaScript rendering, anti-bot bypass, geo-targeting, and multiple
output formats.

This PR includes:
- Provider documentation
(`docs/docs/integrations/providers/zenrows.ipynb`)
- Tool documentation
(`docs/docs/integrations/tools/zenrows_universal_scraper.ipynb`)
- Complete usage examples and API reference links

**Issue:** N/A

**Dependencies:** 
- [langchain-zenrows](https://github.com/ZenRows-Hub/langchain-zenrows)
package (external, available on
[PyPI](https://pypi.org/project/langchain-zenrows/))
- No changes to core LangChain dependencies

**LinkedIn handle:** https://www.linkedin.com/company/zenrows/

---------

Co-authored-by: Mason Daugherty <mason@langchain.dev>

libs/packages.yml

@@ -729,8 +729,10 @@ packages:
 - name: langchain-scrapeless
   repo: scrapeless-ai/langchain-scrapeless
   path: .
+- name: langchain-zenrows
+  path: .
+  repo: ZenRows-Hub/langchain-zenrows
 - name: langchain-oci
   name_title: Oracle Cloud Infrastructure (OCI)
   repo: oracle/langchain-oracle
   path: .
-