HaikuBook: fix warnings generated by Doxygen 1.18.16.

This version generates some warnings and identified some problems in our
documentation. It is also a bit more vigilant about undocumented elements, so
these are now decorated with a brief description. In the BNotification docs I
used a stray \p to create some whitespace between code and an image. This is
now fixed in the CSS instead.

Fix locale docs as suggested by Adrien, more work still needed

A few more small fixes, trying to go quick

Fix a bit of copy pasta in the locale docs

Update locale docs to match new reality.

Created docs for NumberFormat, DateFormat, DateTimeFormat, and TimeFormat
and moved the docs from BLocale to the respective new file. Also
DurationFormat was updated as well.

doxygen once again compiles the docs without warnings.

Fix locale docs as suggested by Adrien, more work still needed

A few more small fixes, trying to go quick

Fix a bit of copy pasta in the locale docs

Update locale docs to match new reality.

Created docs for NumberFormat, DateFormat, DateTimeFormat, and TimeFormat
and moved the docs from BLocale to the respective new file. Also
DurationFormat was updated as well.

doxygen once again compiles the docs without warnings.