Docs refactor (#480)

Big docs refactor! Motivation is to make it easier for people to find
resources they are looking for. To accomplish this, there are now three
main sections:

- Getting Started: steps for getting started, walking through most core
functionality
- Modules: these are different modules of functionality that langchain
provides. Each part here has a "getting started", "how to", "key
concepts" and "reference" section (except in a few select cases where it
didnt easily fit).
- Use Cases: this is to separate use cases (like summarization, question
answering, evaluation, etc) from the modules, and provide a different
entry point to the code base.

There is also a full reference section, as well as extra resources
(glossary, gallery, etc)

Co-authored-by: Shreya Rajpal <ShreyaR@users.noreply.github.com>

docs/modules/prompts/examples/custom_example_selector.md

@@ -0,0 +1,68 @@
+# Create a custom example selector
+
+In this tutorial, we'll create a custom example selector that selects examples every alternate example given a list of examples.
+
+An `ExampleSelector` must implement two methods:
+
+1. An `add_example` method which takes in an example and adds it into the ExampleSelector
+2. A `select_examples` method which takes in input variables (which are meant to be user input) and returns a list of examples to use in the few shot prompt.
+
+Let's implement a custom `ExampleSelector` that just selects two examples at random.
+
+:::{note}
+Take a look at the current set of example selector implementations supported in LangChain [here](../getting_started.md).
+:::
+
+<!-- TODO(shreya): Add the correct link. -->
+
+## Implement custom example selector
+
+```python
+from langchain.prompts.example_selector.base import BaseExampleSelector
+from typing import Dict, List
+import numpy as np
+
+
+class CustomExampleSelector(BaseExampleSelector):
+    
+    def __init__(self, examples: List[Dict[str, str]]):
+        self.examples = examples
+    
+    def add_example(self, example: Dict[str, str]) -> None:
+        """Add new example to store for a key."""
+        self.examples.append(example)
+
+    def select_examples(self, input_variables: Dict[str, str]) -> List[dict]:
+        """Select which examples to use based on the inputs."""
+        return np.random.choice(self.examples, size=2, replace=False)
+
+```
+
+
+## Use custom example selector
+
+```python
+
+examples = [
+    {"foo": "1"},
+    {"foo": "2"},
+    {"foo": "3"}
+]
+
+# Initialize example selector.
+example_selector = CustomExampleSelector(examples)
+
+
+# Select examples
+example_selector.select_examples({"foo": "foo"})
+# -> array([{'foo': '2'}, {'foo': '3'}], dtype=object)
+
+# Add new example to the set of examples
+example_selector.add_example({"foo": "4"})
+example_selector.examples
+# -> [{'foo': '1'}, {'foo': '2'}, {'foo': '3'}, {'foo': '4'}]
+
+# Select examples
+example_selector.select_examples({"foo": "foo"})
+# -> array([{'foo': '1'}, {'foo': '4'}], dtype=object)
+```