If I’m not mistaken the documentation is written in markdown
here is a link to the introduction page
https://github.com/openhab/openhab-docs/edit/main/introduction.md
the file extention is .md
it is just like a readme file on github
you format it just like on the forum here (almost)
heading text
You can edit it right there on github
clone a branch, do your edits, submit PR
Edit:
Here is a thread we used to create the Blockly documentation. The actual documentation in the first post got moved but the first dozen or so posts are Stephen, Rich and Jerome et al discussing how to develop and ultimately merge the documentation