This commit adds copyright attirbutions in SPDX format to files for matrix-bot-go-neb, following the REUSE's specification.

Signed-off-by: Suguru Hirahara <acioustick@noreply.codeberg.org>

This commit adds copyright headers to Markdown, YAML, and labels.j2 files. For the rest of the files, which are ones in YAML and JSON files with the extention ".j2", ".license" files are added following the REUSE's specification.

Signed-off-by: Suguru Hirahara <acioustick@noreply.codeberg.org>