docs: fix typos and stale links

- doc.go: fix 'suppot' typo and double period in 0.3.10 release note
- CONTRIBUTING.md: replace empty email placeholders with GitHub issue
  link (CoC) and reference to SECURITY.md (security reports); correct
  documentation URLs from github.com/darccio/mergo to dario.cat/mergo
- README.md: update Sourcegraph and FOSSA badge URLs from imdario/mergo
  to darccio/mergo

Co-authored-by: Ona <no-reply@ona.com>