Readme: implement alternate way to build EPUB

Workaround for EPUB build Sphinx build:

- no longer use "epub" branch
- remove "hidden" TOC directive on "build" branch via sed
- build on branch
- reset master branch to initial state
- delete temporary branch

Signed-off-by: Olivier Carrère <carrereo@gmail.com>
Acked-by: Angela Cristina <acristina@redaction-technique.org>