- Update Makefile and define DOCSDIR variable
- Add `man` sphinx build tag to the `man` make-target
- Exclude `_man.rst` if the `man` build tag is set
- Revert b45e685

- Fix channel ID lookup
- Fix validation schema on missing data
- Add more stream types to live streams: HTTPStream for content
  restriction clips, and DASHStream where HLS is not available
- Apply minor code refactoring

Installing python header files is only relevant when building
dependencies with Python C extensions from source, like lxml or
pycryptodome, which is out of scope of the Streamlink docs.

The docs should be as explicit as possible, but this might confuse
inexperienced users, so an explanation might be useful.

Twitch doesn't include duration data for prefetch segments, which means
when more than one prefetch segment is included in the HLS playlist,
the start time of the second (or third) prefetch segment has to be
guessed from the duration of the regular segments. This is done by
calculating the average of all available regular segments and using that
as an offset of the last regular segment for each prefetch segment.

This however can cause issues when segment durations vary a lot and the
start time of an annotated ad block doesn't match the extrapolated start
time of the first ad segment due to the miscalculation. The ad segment
then gets incorrectly included in the output stream.

Since a discontinuity tag is always present, even between prefetch tags
with ads, we can make use of it and treat prefetch segments after a
discontinuity tag as ads. This won't cause any issues during the
transition to the regular stream content, as it only affects the logic
of prefetch segments.