Update docs and tutorials to place `llama2-chat` and `codellama-instruct` in the correct directory

[ThorstenHans]

As discussed in the following issues, this PR updates the documentation and instructs users to place llama2-chat and codellama-instruct models at .spin/ai-models/llama/ instead of .spin/ai-models/

• https://github.com/fermyon/spin/issues/2467
• https://github.com/fermyon/developer/issues/1260

closes #1260

Pre-Merge Content Checklist

This documentation has been checked to ensure that:

• [x] The title, template, and date are all set
• [x] Does this PR have a new menu item (anywhere in templates/*.hbs files) that points to a document .md that is set to publish in the future? If so please only publish the .md and .hbs changes in real-time (otherwise there will be a menu item pointing to a .md file that does not exist)
• [x] File does not use CRLF, but uses plain LF (hint: use cat -ve <filename> | grep '^M' | wc -l and expect 0 as a result)
• [x] Has passed bart check
• [x] Has been manually tested by running in Spin/Bartholomew (hint: use PREVIEW_MODE=1 and run npm run styles to update styling)
• [x] Headings are using Title Case
• [x] Code blocks have the programming language set to properly highlight syntax and the proper copy directive
• [x] Have tested with npm run test and resolved all errors
• [x] Relates to an existing (potentially outdated) blog article? If so please add URL in blog to point to this content.