doc: extract_content.py: Detect included files in a more robust way

Allow '.. <figure/include/image/...>:: <path>' to appear anywhere within
a line, and find multiple directives on a single line. This is needed to
find files included e.g. within tables.

Implemented by making the <path> part of the regex more specific and
searching for matches anywhere within the contents of the file. Should
be a bit faster too.

Maybe there's some tiny potential for false positives, but this
generates the same file list as the old version for the current docs at
least.

Fixes: #21466

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>