Update namespace documentation

[ahouseholder]

This PR:

• resolves Reserve registered namespace invalid #907
• resolves [Documentation] Clarify reserved special purpose namespaces #908
• might address [Documentation] "unregistered" vs. "private" #909 but there is not enough detail in that issue to tell whether it does or not
• Explicitly calls out RESERVED namespace strings:
  • invalid and x_invalid are blocked from use
  • example and x_example are reserved for examples and documentation
  • test and x_test are reserved for tests
• adds a nist namespace
• refactors one of the probability-based decision points based on NIST 800-30 into the nist namespace
  • this incidentally resolved a situation in which two identically named decision points were clobbering each other when we generated the .json files, hence what appears to be an unrelated change to data/json/decision_points/basic/probability_scale_in_5_weighted_levels_ascending_1_0_0.json
• revises the namespaces.md documentation diagrams for clarity

Copilot Summary

This pull request refactors the probability scale decision point definitions and updates the documentation for namespace structure and extension usage. The main changes are the migration of the NIST-based probability scale to its own namespace, a redesign of the "basic" weighted probability scale, and comprehensive improvements to the documentation on namespace conventions and extensions.

Decision Point Refactoring:

• Migrated the NIST SP 800-30 probability scale (P_5X) from the basic namespace to a new nist#800-30 namespace, restoring its original value definitions and associated registry entries. [1] [2]
• Redesigned the basic probability scale (P_5W) to use ascending probability ranges with higher resolution as probability increases, replacing the previous NIST-based value definitions.
• Updated the SSVC object registry to remove the old basic.P_5X entry and add the new nist#800-30.P_5X entry. [1] [2]
• Updated the Python module src/ssvc/decision_points/basic/probability/__init__.py to remove the NIST probability scale from the basic namespace and ensure only the correct decision points are registered.

Documentation Improvements (Namespace and Extension Structure):

• Expanded documentation in docs/reference/code/namespaces.md to clarify reserved namespace strings, the use and requirements of fragments in registered and unregistered namespaces, and the semantic and structural requirements for namespace extensions. This includes new diagrams, notes, and examples for better understanding. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]

These changes improve the clarity and maintainability of the decision point definitions and provide more robust guidance for namespace usage and extension in SSVC.

[ahouseholder]

Screenshots of the most significant content changes:

Note

The following screenshot is slightly behind the current branch. The later version has been copy edited to fix the example and test lines.

[sei-vsarvepalli]

also has other probability stuff updates in it - all okay.