Automate the generation of available model information for the guides.

[keithrbennett]

It would be helpful to have up-to-date easy-to-digest documentation informing as to which models are available for use, along with useful information about them.

PR #71 proposed a rake task that would generate a markdown page in the guides that would have this information.

The output consists of text other than the tables, but here is an excerpt from a table (the full output can be seen at docs/guides/available-models.md.):

Chat Models (96)

ID	Type	Name	Provider	Context	MaxTok	Family	In$/M	Out$/M
aqa	chat	Model that performs Attributed Question Answering.	gemini	7168	1024	aqa	0.00	0.00
babbage-002	chat	Babbage 002	openai	16385	16384	babbage	0.50	1.50
chat-bison-001	chat	PaLM 2 Chat (Legacy)	gemini	4096	1024	other	0.08	0.30
chatgpt-4o-latest	chat	ChatGPT-4o Latest	openai	128000	16384	gpt4o	0.50	1.50

[crmne]

This is awesome @keithrbennett. Thank you for this!