update documentation users/rmg/thermo.rst

add some images too.
move general info on RMG databases to dedicated introduction.rst file

instead of having it inside kinetics.rst.
This info also applied to thermochemistry.
update users/rmg/database/thermo.rst documentation
add parts on symmetry, chirality, QMTP file types in thermo.rst docs
add thermoEstimation module documentation

copy QMTP part in users/rmg/input.rst into the module
documentation of the thermoEstimator.
fix some minor errors in thermo docs
Redirect documentation link on heirarchical tree structures.