Move supporting matrix generator assets out of eng/common (#5027)

This PR contributes to addressing:
- #4995 

by moving out of `eng/common/scripts/job-matrix` assets that should not be mirrored to language repos. These assets include documentation, samples, and tests.

## Changes made

- Moved contents of `eng/common/scripts/job-matrix/README.md` to newly introduced `doc/common/matrix_generator.md`;
- Replaced contents of `eng/common/scripts/job-matrix/README.md` with a pointer to `doc/common/matrix_generator.md`;
- Moved contents of `eng/common/scripts/job-matrix/scripts` and `eng/common/scripts/job-matrix/tests` to corresponding directories in newly introduced directory of 
`eng/common-tests/matrix-generator`.

Note the introduced directory and doc file use `matrix generator` as name, not `job matrix`. This matches the terminology used in the documentation.

## Changes NOT made

- The contents of files moved from `scripts` and `tests` directories have not been updated at all. The files have been moved unchanged.
- The contents of the moved `README.md` have not been updated, even though they should, as they refer to paths that have changed as a result of the move. To keep the PR diff clean, I will instead update them in a separate PR:
  - #5023