Llms txt for website

[Jolanrensen]

Follow up for #1615 and related to #1258.

Required for: Kotlin/kotlin-agent-skills#48

Enables llms-txt in WriterSide. This generates a txt file for each topic in the Writerside _llms folder.
It also generates an llms.txt file containing everything in one file.
However, this is probably too large for most LLMs to parse, so similar to how kotlinlang.org uses a script to
create an index-like llms.txt https://github.com/JetBrains/kotlin-web-site/blob/master/scripts/llms/generate-llms-index.mjs
I (and Junie) did a similar thing.
Using scripts/llms/generate-llms-index.mjs, our GitHub action now renames the big llms.txt file to llms-full.txt
and generates a new llms.txt serving as a table-of-contents for all generated llm files.

For example:
llms.txt
llms-full.txt
_llms/add.txt
...

[Jolanrensen]

While I could test the script works locally, I cannot check it works well with gh actions yes. I will merge and try :)