docs: add llms.txt file

[mateuszlampert]

Description

This PR adds llms.txt file to our docs - llms.txt is built automatically using docusaurus-plugin-llms-txt plugin. Because of some bug regarding absolute urls in the plugin used, this PR also adds postbuild script that fixes duplicated url slugs in the llms.txt file.

Introduces a breaking change?

• Yes
• No

Type of change

• Bug fix (change which fixes an issue)
• New feature (change which adds functionality)
• Documentation update (improves or adds clarity to existing documentation)
• Other (chores, tests, code style improvements etc.)

Tested on

• iOS
• Android

Testing instructions

• build docs locally (yarn build -> yarn serve)
• verify /react-native-executorch/llms.txt url

Screenshots

llms.txt

Related issues

#416

Checklist

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have updated the documentation accordingly
• My changes generate no new warnings

Additional notes

After merging this PR, our DocSearch crawler configuration should be updated, so is does not index llms.txt file.

[msluszniak]

Works for me: