Add documentation about versioning system

[mithro]

The documentation should include information about how the PDK is versioned.

See related issue at https://github.com/google/open-source-pdks/issues/8

[mohanad0mohamed]

Hi @mithro

There is a file in the documentation which includes information about how the PDK is versioned.

Kindly see the file at : https://github.com/google/gf180mcu-pdk/blob/main/docs/versioning.rst

[mithro]

This will be fixed by https://github.com/google/gf180mcu-pdk/pull/24