docs: add llms.txt for LLM-assisted discovery

Adds a llms.txt file (per the emerging llmstxt.org convention) listing
the published User Manual pages, so LLM tools and AI coding assistants
can quickly find canonical tealdeer documentation instead of guessing
or hallucinating usage/config details.

docs/llms.txt

@@ -0,0 +1,23 @@
+# Tealdeer
+
+> Tealdeer is a very fast implementation of [tldr](https://github.com/tldr-pages/tldr) in Rust: simplified, example-based, community-driven man pages, accessed through the `tldr` binary.
+
+## Getting Started
+
+- [Introduction](https://tealdeer-rs.github.io/tealdeer/intro.html): What tealdeer is and how it relates to the tldr-pages project.
+- [Installing](https://tealdeer-rs.github.io/tealdeer/installing.html): Installation methods, including package managers.
+- [Usage](https://tealdeer-rs.github.io/tealdeer/usage.html): Basic command-line usage of the `tldr` binary.
+
+## Configuration
+
+- [Configuration](https://tealdeer-rs.github.io/tealdeer/config.html): Overview of the TOML config file.
+- [Section: display](https://tealdeer-rs.github.io/tealdeer/config_display.html): Output format settings, including pager usage.
+- [Section: style](https://tealdeer-rs.github.io/tealdeer/config_style.html): Customizing colors and text styling.
+- [Section: search](https://tealdeer-rs.github.io/tealdeer/config_search.html): Page search and cache lookup settings.
+- [Section: updates](https://tealdeer-rs.github.io/tealdeer/config_updates.html): Automatic and manual cache update behavior.
+- [Section: directories](https://tealdeer-rs.github.io/tealdeer/config_directories.html): Overriding cache and config directory paths.
+
+## Advanced
+
+- [Custom Pages and Patches](https://tealdeer-rs.github.io/tealdeer/usage_custom_pages.html): Writing custom pages and patching existing ones.
+- [Tips and Tricks](https://tealdeer-rs.github.io/tealdeer/tips_and_tricks.html): Example use cases, such as showing a random page on shell start.