C++ improvements

[markkohdev]

Description

In this PR we will:

• Clean up the C++ module structure
• Add CMake for standalone C++ compilation
• Improve formatting within the c++ module
• Add doctest testing framework for C++ unit tests
  • Note: Right now we have a single test. In order to properly utilize DocTest we need to first provide namespacing for this library as per cpp wrappers are using default namespace #52 -- otherwise we run into issues with ambiguous function calls during doctest stringification.
• Update documentation to be more comprehensive around building, testing, etc.
• Add a github pull request template :)

Changes Made

C++

Main changes:

• Break cpp dir into src and test directories
• Add cmake and documentation around it
• Add an initial test
• Run clang-format with new 120 line length standard

Python

• Remove c++ formatting of python bindings from tox
• Remove voyager-headers link since it doesn't seem to be needed
• Update location of C++ files

Java

• Update location of C++ files
• Run formatter
• Update docs

Testing

Checklist

• My code follows the code style of this project.
• I have added and/or updated appropriate documentation (if applicable).
• All new and existing tests pass locally with these changes.
• I have run static code analysis (if available) and resolved any issues.
• I have considered backward compatibility (if applicable).
• I have confirmed that this PR does not introduce any security vulnerabilities.

Additional Comments

[markkohdev]

softlink to allow the clang-format github action to work correctly

[psobot]

LGTM overall, but if we're adding C++ tests (even just the one) in this PR, IMO we should add them to the GitHub Actions workflow so they run on CI before merging this.

[psobot]

Suggested change

	this will update the java documentation located in [docs/java/](https://github.com/spotify/voyager/tree/main/docs/java).
	This will update the java documentation located in [docs/java/](https://github.com/spotify/voyager/tree/main/docs/java).

[alexander-riss]

without any pressure - is there a timeframe for merging this? just asking for our own planning on when to start integrating these changes

[alexander-riss]

just a friendly ping - any update on this? do you think it makes sense for potential users to start picking up your branch directly instead of waiting for this to be merged? @markkohdev