Update llms.txt documentation with improved clarity and structure

Summary

This PR restructures the llms.txt documentation page to improve clarity and add information about recent platform capabilities. The changes convert bullet-point descriptions to flowing prose, clarify the availability of each file type, add analytics information, and address PR feedback about content placement.

Key changes:

• Restructured content from bullet points to prose for better readability
• Clarified that llms.txt is available at any level of documentation hierarchy (e.g., /llms.txt, /docs/llms.txt, /v1/api-reference/llms.txt)
• Clarified that llms-full.txt is only available at the root of documentation sites
• Added explanation of automatic markdown serving when LLM bots are detected (90%+ token reduction)
• Moved image to appear after the llms.txt section (per PR feedback from @kgowru)
• Made example links clickable (converted from code blocks to markdown links)
• Added "Analytics and monitoring" section describing dashboard features
• Added "LLM" to Vale acronyms exceptions to fix linting

Review & Testing Checklist for Human

• Verify availability claims: Test that llms.txt is actually accessible at different documentation levels (root, sub-sections, API reference pages) and that llms-full.txt is only at the root. Try accessing both files on a live docs site.
• Verify analytics accuracy: Confirm that the analytics features described (LLM provider tracking, bot vs. human traffic breakdown) actually exist in the Fern Dashboard and match the description.
• Check content flow: Review whether the restructured prose reads better than the previous bullet-point format. Consider if any important context was lost (like "Small and fast", "Token efficient" descriptors).
• Test example links: Click through the example links to docs.cohere.com and buildwithfern.com to verify they work and display the expected content.

Test Plan

1. Navigate to a Fern docs site (e.g., docs.cohere.com) and try accessing /llms.txt, /docs/llms.txt (if it has sub-sections), and /api-reference/llms.txt to verify availability
2. Try accessing /llms-full.txt at root and at a sub-path to confirm it's only available at root
3. Check the Fern Dashboard analytics to verify the described features exist

Notes

• This documentation update is based on recent platform improvements discussed with Kapil (@devalog)
• Addresses PR feedback from @kgowru about moving the markdown serving statement to the llms.txt section and moving the image up
• Session: https://app.devin.ai/sessions/83af156cfd684cb18bfc587005ed53bd
• The 90%+ token reduction claim for markdown serving should be verified against actual metrics if available