docs: Add documentation for GitHub Actions and Docker image builds

[anayden]

Added some documentation on github actions. Please suggest where a link to the file would be appropriate.

[github-actions]

Docker image tag(s) pushed:

rnacentral/r2dt:pr-169

Labels added to images:

org.opencontainers.image.created=2024-12-21T15:27:33.023Z
org.opencontainers.image.description=Visualise RNA secondary structure in consistent, reproducible and recognisable layouts
org.opencontainers.image.licenses=Apache-2.0
org.opencontainers.image.revision=6f70108eddf07746854e637b7f47ead81568ce2b
org.opencontainers.image.source=https://github.com/r2dt-bio/R2DT
org.opencontainers.image.title=R2DT
org.opencontainers.image.url=https://github.com/r2dt-bio/R2DT
org.opencontainers.image.version=pr-169