github: Remove git submodules

[runleveldev]

Issue: Remove Git submodules and simplify dependency management

Description:
The project currently uses Git submodules to manage external code. This adds maintenance overhead and complicates updates or cloning workflows.

Recommendation:

• Vendor critical or stable dependencies directly into the repository where feasible.
• For actively maintained external projects, reference their upstream Git repositories or package manager sources (e.g., npm, pip) instead of submodules.

Benefits:

• Simplifies repository cloning and CI/CD setup
• Eliminates manual submodule initialization and update steps
• Ensures automatic access to upstream updates and security patches
• Improves build reproducibility and reduces contributor friction

[runleveldev]

Is this a good idea? I don't like submodules since they don't keep the linked repo up-to-date and some of the ones we have (like the landing page) should be directly part of this repo. This would let us update all URLs for example in one PR rather than distributing them to disparate repos.