ci: doc-build: skip Kconfig docs build on pull requests

The documentation supports a special target named "html-fast" that skips
generation of all Kconfig pages. Instead, it creates a single dummy page
where a reference to all existing Kconfig options is placed. This means
that references are resolved, but content is not rendered. Since Kconfig
help is rendered as a literal, chances of breaking documentation
build due to Kconfig changes should be low. The change proposed in this
patch should speed up documentation build on pull requests while a
proper solution is found for the Kconfig docs.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>