Outdated links to Doxygen

A few months ago, the documentation was moved:

- https://www.openfoam.com/documentation/cpp-guide/html/
+ https://www.openfoam.com/documentation/guides/latest/doc/

However, the links (also in v2112) are still pointing to the old one, in these files:

• https://develop.openfoam.com/Development/openfoam/-/blob/master/etc/controlDict#L29
• https://develop.openfoam.com/Development/openfoam/-/blob/master/tutorials/incompressible/pimpleFoam/LES/decayIsoTurb/README.md#L13
• https://develop.openfoam.com/Development/openfoam/-/blob/master/tutorials/incompressible/simpleFoam/turbulentFlatPlate/README.md#L16
• https://develop.openfoam.com/Development/openfoam/-/blob/master/tutorials/verificationAndValidation/schemes/divergenceExample/README#L17

This issue could also be read as "the automatic redirection of the website does not work for specific pages", as the link to, e.g., postProcess_8C.html just redirects to the main page.

By the way, awesome feature of providing a -doc flag, as in postProcess -doc!

(I am not sure if I could have even directly contributed code here, as this is a trivial fix)