Remove fixtures references

[smaye81]

This removes the concept of fixtures from the repo by doing the following:

• Remove all fixture files from packages/test-fixtures.
• Rename packages/test-fixtures to packages/proto.
• Update all references to test-fixtures to use proto.
• Generate descriptorSet for the helper in packages/plugin in that directory rather than using an external package as a dep.
• Update docs

Note: as a follow on, we can convert usages of protoc to buf generate or buf build.

[timostamm]

🎉 just one nit about formatting in the manual.

[timostamm]

LGTM!