doc: llext: split existing text into separate files

The existing index.rst file contains a mix of information about the
llext subsystem and the LLEXT Extension Development Kit (EDK). This
commit splits the existing text into two separate files, index.rst and
build.rst, as a first step on improving the documentation.

No change in text is done in this commit.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>