Rename `source_code_be` to `create_source_code_management` in the docs

Move the rename of backend\source_code_be to create_source_code_management

The following discussion from !139556 (merged) should be addressed:

• @msedlakjakubowski started a discussion: (+3 comments)

  @sean_carroll When deleting a page's contents, we've got to set up a redirect there instead, so we don't give a 404: https://docs.gitlab.com/ee/development/documentation/redirects.html#redirect-to-a-page-that-already-exists

  Where should this redirect to?