Improve documentation build logic

[moe-ad]

Description of the modifications

While working on #490 and ansys-internal/pyansys-planning#96, it was observed that the build process of the documentation can be improved. Currently, pre-build scripts and build commands are being executed through tox.

However, best practice requires code related to documentation build to be located within the conf.py file and "sphinx build ..." command is preferred to the "make ..." command since the former is OS agnostic unlike the latter (which is what is currently being used through tox).

Useful links and references

The following is recommended:

• The code in the pre-build script should be moved to conf.py.
• Conf.py should be modified accordingly to run this code.
• Documentation build should be done with "sphinx build " command.