Enable llms.txt generation in docs

[koperagen]

Related to #1258
More info about this file: https://www.jetbrains.com/help/writerside/generate-llms-txt.html
I couldn't test it locally, but in GH Actions use writerside builder 2025.04.8412
that includes this feature: https://www.jetbrains.com/help/writerside/8312.html

Let's see the content, decide if we need it. Although single artifact for LLM is a thing, LLM providers seem to use our *.md topics from GitHub instead of documentation website.

[Jolanrensen]

llms.txt

this is what it spits out :)

While it looks rather complete, I am not sure if it may be too large for users to include in their context... Maybe we could try <llms-text/> which also produces individual text files:

_llms.zip

[koperagen]

Hmm.. i don't know. I'm yet to fully grasp the idea, need to read more https://llmstxt.org/