graph: move doc to graph.h and graph.c

Move the documentation from Documentation/technical/api-history-graph.txt to
graph.h and graph.c as it's easier for the developers to find the usage
information beside the code instead of looking for it in another doc file.

The graph library was already well documented, so few comments were added to
both graph.h and graph.c

Also documentation/technical/api-history-graph.txt is removed because
the information it has is now redundant and it'll be hard to keep it up to
date and synchronized with the documentation in the header file.

Signed-off-by: Heba Waly <heba.waly@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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