MO specific documentation section

[balazagi]

identifying and adding any MO specific instructions to a separate section in the documentation titled

• "Met Office specific instructions"

[zmaalick]

associated branch: feature_update_documentation_i39

[balazagi]

The only MO specific instructions were:

• use of local version of the Improver library (needed for the dewpoint calculation function) due to an existing bug

• use of Ascend/shape function needed for the extract_rot_cube() function

As discussed since both of these functions are to be removed/replaced then there is no need to update documentation for these. So closing this issue.