docs: add Linkup integration documentation (#28366)

## Description

First of all, thanks for the great framework that is LangChain!

At [Linkup](https://www.linkup.so/) we're working on an API to connect
LLMs and agents to the internet and our partner sources. We'd be super
excited to see our API integrated in LangChain! This essentially
consists in adding a LangChain retriever and tool, which is done in our
own [package](https://pypi.org/project/langchain-linkup/). Here we're
simply following the [integration
documentation](https://python.langchain.com/docs/contributing/how_to/integrations/)
and update the documentation of LangChain to mention the Linkup
integration.

We do have tests (both units & integration) in our [source
code](https://github.com/LinkupPlatform/langchain-linkup), and tried to
follow as close as possible the [integration
documentation](https://python.langchain.com/docs/contributing/how_to/integrations/)
which specifically requests to focus on documentation changes for an
integration PR, so I'm not adding tests here, even though the PR
checklist seems to suggest so. Feel free to correct me if I got this
wrong!

By the way, we would be thrilled by being mentioned in the list of
providers which have standalone packages
[here](https://langchain-git-fork-linkupplatform-cj-doc-langchain.vercel.app/docs/integrations/providers/),
is there something in particular for us to do for that? 🙂

## Twitter handle

Linkup_platform
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libs/packages.yml

@@ -153,3 +153,6 @@ packages:
   - name: langchain-neo4j
     repo: langchain-ai/langchain-neo4j
     path: libs/neo4j
+  - name: langchain-linkup
+    repo: LinkupPlatform/langchain-linkup
+    path: .