docs: add llms.txt and llms-full.txt for v5 documentation

[tamish560]

What

Adds llms.txt and llms-full.txt to the repository root following the llmstxt.org specification.

Why

AI coding assistants (Cursor, Copilot, Claude, etc.) frequently hallucinate Echo v4 syntax when users are working with v5. The v4-to-v5 breaking changes are significant (Context is now a struct, handlers use *echo.Context, slog replaces the custom Logger, HTTPErrorHandler params swapped, etc.) and LLMs don't have accurate v5 context.

llms.txt provides a concise summary with quick-reference patterns. llms-full.txt provides the complete v5 API surface including all breaking changes, method signatures, and migration checklist.

Contents

llms.txt - Summary file with:

• Project overview and key v5 facts (import path, handler signature, Go version)
• Quick reference code patterns (server setup, routing, binding, errors, logging, middleware)
• v4-to-v5 migration checklist (14 items)
• Links to detailed docs

llms-full.txt - Full reference with:

• All critical v5 breaking changes with before/after code
• Complete Echo struct and methods
• Complete Context methods
• All generic helper functions (PathParam, QueryParam, FormValue, etc.)
• Group methods, middleware list, binding functions
• PathValues type, echotest package, StartConfig
• Full server example with graceful shutdown

Closes #2980 (discussion proposal by @iharshr, green-lit by @aldas).

Notes

• All code examples verified against API_CHANGES_V5.md and actual source files (echo.go, context.go, group.go, middleware/)
• Follows the llmstxt.org format (H1 title, blockquote summary, H2 sections with links)
• No code changes, documentation only

[aldas]

Should it concentrate so much on migration? would not v5 description be enough?

[tamish560]

good point. the llms.txt is meant for AI assistants to understand the framework, not for human migration. I'll rework both files to describe v5 as it is and drop the v4 comparisons and migration checklist. the "v5 vs v4" annotations and the migration section at the bottom can go.

the llms-full.txt has the same issue with the "Critical v5 Breaking Changes" section being framed entirely as v4-to-v5 diffs. I'll rewrite that to just describe the v5 API without the v4 references.

give me a bit to update both files.