docs: align custom_tool document headers (#13252)
On the [Defining Custom
Tools](https://python.langchain.com/docs/modules/agents/tools/custom_tools)
page, there's a 'Subclassing the BaseTool class' paragraph under the
'Completely New Tools - String Input and Output' header. Also there's
another 'Subclassing the BaseTool' paragraph under no header, which I
think may belong to the 'Custom Structured Tools' header.
Another thing is, there's a 'Using the tool decorator' and a 'Using the
decorator' paragraph, I think should belong to 'Completely New Tools -
String Input and Output' and 'Custom Structured Tools' separately.
This PR moves those paragraphs to corresponding headers.
