Change makefile to generate an automatic version for the baseset and newgrfs

[zephyris]

Versions were hard-coded in various places, problematic as overlooking one can mess up versioning. Versioning used an integer version for the internal version of basesets and NewGRFs (1, 2, 3, ..., 8), and a user-facing semantic version (0.1, 0.2, 0.3, ..., 0.8).

This series of changes:

1. Expands the makefile to generate an automatic and continuously increasing internal version, using days past 2000-01-01. Additionally, copies the logic from OpenGFX(1) to record repo version data in makefile.vcs, which can be used as a dependency for outputs which depend on these versions for naming.
2. Updates the baseset obg generating Python script to use the internal and user-facing versions passed from the makefile.
3. [Fixes a bug in the nml preprocessing Python script]
4. Updates the NewGRF nml preprocessing Python script to use the internal version passed from the makefile.
5. Updates the release docs given that internal version is now automatic, and a user-facing semantic version now only needs to be set in one place in the makefile.

The makefile variables and rules for the versions used by the CDN upload prep rules are unchanged.