[skip changelog] Add documentation for gRPC Debug service

[per1234]

Please check if the PR fulfills these requirements

• The PR has no duplicates (please search among the Pull Requests before creating one)
• The PR follows our contributing guidelines

• What kind of change does this PR introduce?

Add explanatory comments for the Debug service of the gRPC interface to the source code. These comments will be included in the generated gRPC interface documentation.

• What is the current behavior?

There is insufficient explanation of the gRPC interface in the documentation.

• What is the new behavior?

Improved documentation of the gRPC interface.

• Does this PR introduce a breaking change?

No

[per1234]

I rebased to fix the merge conflict and added documentation for the new import_dir field:

    // Directory containing the compiled executable. If `import_dir` is not
    // specified, the executable is assumed to be in
    // `{sketch_path}/build/{fqbn}/`.
    string import_dir = 8;