Document for a custom prompt template is outdated.
Currently, the document on [Creating a custom prompt template](https://langchain.readthedocs.io/en/latest/modules/prompts/examples/custom_prompt_template.html) is outdated, as the code in the guide is no longer functional for the following reasons:

1. The document outlines creating a custom prompt template by inheriting from `BasePromptTemplate` with only the `format` method. However, a new required method `format_prompt` has been introduced as an interface for adapting chat-style prompt template usage.
2. Maybe, the `StringPromptTemplate` was created to absorb the change, but it is currently not exposed.

Therefore, I suggest using `StringPromptTemplate` instead of `BasePromptTemplate`, and exposing it in `langchain.prompts`.

I have created a [PR#1753](https://github.com/hwchase17/langchain/pull/1753) for this, and would appreciate it if you could review it.

Additionally, I have created another PR to slightly modify the class docstring for both `BasePromptTemplate` and `StringPromptTemplate`, as their current docstrings are outdated and require updating in relation to the issue at hand.