First batch of GDI SOP documentation

[M-casado]

Along the shift from Google Docs to GitHub, I grouped in this PR three main changes:

• ZenHub#255: Define object accessioning proposal in GitHub. This proposal is up to review, given that there are some changes, out of which I would highligh:
  • Filenames of GDI SOPs only contain the base identifier (e.g., GDI-SOP0001_<title>.md), rather than the version or node's code (e.g., GDI-SOP0001.v1_SWE.v1_<title>.md). See details within the accessioning proposal.
• ZenHub#256: Create GitHub issue template: request new SOP.
  • Through this quick and user-friendly template, anyone can request the creation of a new SOP without knowing technical details nor how to use GitHub (beyond creating an issue, of course).
• ZenHub#262: Add General SOP template to GitHub repo. Highlighted changes:
  • I modified some parts of the template, added more descriptions and examples.
  • I added a metadata section at the beginning, where we can track the metadata of the files, rather than in the filenames.

[bhendriksze]

Seems to cover all that was discussed before. No review comments.