Add optional base_url arg to GitbookLoader (#1552)

First of all, big kudos on what you guys are doing, langchain is
enabling some really amazing usecases and I'm having lot's of fun
playing around with it. It's really cool how many data sources it
supports out of the box.

However, I noticed some limitations of the current `GitbookLoader` which
this PR adresses:

The main change is that I added an optional `base_url` arg to
`GitbookLoader`. This enables use cases where one wants to crawl docs
from a start page other than the index page, e.g., the following call
would scrape all pages that are reachable via nav bar links from
"https://docs.zenml.io/v/0.35.0":

```python
GitbookLoader(
    web_page="https://docs.zenml.io/v/0.35.0", 
    load_all_paths=True,
    base_url="https://docs.zenml.io",
)
```

Previously, this would fail because relative links would be of the form
`/v/0.35.0/...` and the full link URLs would become
`docs.zenml.io/v/0.35.0/v/0.35.0/...`.

I also fixed another issue of the `GitbookLoader` where the link URLs
were constructed incorrectly as `website//relative_url` if the provided
`web_page` had a trailing slash.

langchain/document_loaders/gitbook.py

@@ -12,9 +12,26 @@ class GitbookLoader(WebBaseLoader):
     2. load all (relative) paths in the navbar.
     """
 
-    def __init__(self, web_page: str, load_all_paths: bool = False):
+    def __init__(
-        """Initialize with web page and whether to load all paths."""
+        self,
+        web_page: str,
+        load_all_paths: bool = False,
+        base_url: Optional[str] = None,
+    ):
+        """Initialize with web page and whether to load all paths.
+
+        Args:
+            web_page: The web page to load or the starting point from where
+                relative paths are discovered.
+            load_all_paths: If set to True, all relative paths in the navbar
+                are loaded instead of only `web_page`.
+            base_url: If `load_all_paths` is True, the relative paths are
+                appended to this base url. Defaults to `web_page` if not set.
+        """
         super().__init__(web_page)
+        self.base_url = base_url or web_page
+        if self.base_url.endswith("/"):
+            self.base_url = self.base_url[:-1]
         self.load_all_paths = load_all_paths
 
     def load(self) -> List[Document]:
@@ -24,7 +41,7 @@ class GitbookLoader(WebBaseLoader):
             relative_paths = self._get_paths(soup_info)
             documents = []
             for path in relative_paths:
-                url = self.web_path + path
+                url = self.base_url + path
                 print(f"Fetching text from {url}")
                 soup_info = self._scrape(url)
                 documents.append(self._get_document(soup_info, url))