Split data-semantic-conventions.md into multiple files

[Oberon00]

I think that the semantic conventions should probably be split into different files per topic, e.g. data-span-http.md, data-resources, etc. With incoming PRs such as #263 and #303 the file would grow beyond reasonable size. However, that should probably be done after the aforementioned PRs are merged, as such file-splitting actions tend to cause a lot of merge conflicts.