Bundler design document

Design document that covers the planned implementation of Bundler¹
support in Cachi2. Note that this design doc draws inspiration for the
implementation done in Cachi2, but takes a different approach when it
comes to providing the content for performing the hermetic builds and
for the SBOM data generation.

Since design documents are meant to help all contributors to understand
the implementation and logic behind any feature, we have decided to add
them as part of the repository's documentation, starting from this one.

¹ https://bundler.io/

Signed-off-by: Michal Šoltis <[email protected]>
Co-authored-by: Bruno Pimentel <[email protected]>