Not experienced with Zeal, but I have used devdocs, which is open source. Devdocs parses the relevant documentation pages and stores them in a more structured way inside a browser database file. I would prefer to keep that in a local SQLite file I fully control. However, storing the data is not enough, I need an easy way to search and browse the documentation. Datasette is a Python project which spawns a web interface to a SQLite database which gives an interactive way to explore the data. This interface should be sufficient for locally hosting and searching the documentation.