Add autogeneration of llms.txt during mkdocs build; add docs page on using llms.txt (google#1023)

* Add mkdocs-llmstxt and related config

* Remove existing llms.txt and llms-full.txt from repo

* Add docs page for llms.txt

* Minor edit

* Minor edit

* Add *.md to glob syntax in mkdocs-llmstxt config

* Add explicit URLs in API reference section to avoid build warnings

* Fix all build warnings related to bad cross-links and incorrect anchor links

* Suppress INFO logs about absolute links - the CI link checker will catch any issues

* Fix remaining build warnings

* Add Gemini CLI instructions

* Fix link to bidi tutorial

Author: koverholt

docs/agents/config.md

@@ -51,7 +51,7 @@ To setup ADK for use with Agent Config:
 1.  Install the ADK Python libraries by following the
     [Installation](/adk-docs/get-started/installation/#python)
     instructions. *Python is currently required.* For more information, see the
-    [Known limitations](?tab=t.0#heading=h.xefmlyt7zh0i).
+    [Known limitations](#known-limitations).
 1.  Verify that ADK is installed by running the following command in your
     terminal: