Add Tiktokenizer link in "How to count tokens"

[EliahKagan]

This adds a link to the popular Tiktokenizer webapp, in the section of the "How to count tokens with tiktoken" notebook that talks about the OpenAI Tokenizer. It retains the reference to the OpenAI Tokenizer as well.

Rationale:

• The tiktoken FAQ recommends Tiktokenizer as a resource, in the first line under "Usage help". Besides indicating that it is a helpful resource, this also suggests that OpenAI is already willing to direct users to it.
• Tiktokenizer supports some important encodings the OpenAI Tokenizer currently does not, such as cl100k_base. This allows users to see how text is tokenized for chat models like gpt-3.5-turbo and gpt-4, and for the embedding model text-embedding-ada-002. In contrast, the OpenAI Tokenizer currently only supports GPT-3 and Codex.

(I am not affiliated in any way with Tiktokenizer. I learned about it a while ago from the tiktoken FAQ, and I've often found it useful.)

I'm unsure what the best wording is here, but I phrased it "or the third-party Tiktokenizer webapp" to so no readers are misled into thinking Tiktokenizer is itself developed by OpenAI.

[ted-at-openai]

Good suggestion. Asking the owner of tiktokenizer here: dqbd/tiktokenizer#13

[EliahKagan]

Good idea--thanks! (I look forward to hearing how the author feels about it.)

[dqbd]

Hey! Glad to see that people find Tiktokenizer still useful and I'm fine with keeping the website alive for the foreseeable future as well as keeping the reference to the tool in the guide.