docs(assets): Add SVG logo and ICO favicon.

Adds the SVG logo I created (composited rather) in mid-feb in one of my PRs that went slightly off-topic about migrating docs from Github Wiki.

Enables the logo and favicon in `mkdocs.yml`. SVG sources for both full colour and simplified monochromatic are included along with PNG images for use anywhere else, such as the organization logo.

The two SVG of the same names from `src` dir, are used by the docs and have been processed through SVGOMG, an SVG file size optimizer. Any future modifications should use the source files.

docs/mkdocs.yml

@@ -6,8 +6,11 @@ repo_url: 'https://github.com/docker-mailserver/docker-mailserver'
 copyright: '<p>&copy <a href="https://github.com/docker-mailserver"><em>Docker Mailserver Organization</em></a><br/><span>This project is licensed under the MIT license.</span></p>'
 
 docs_dir: 'content/'
+site_url: 'https://docker-mailserver.github.io/docker-mailserver'
 theme:
   name: 'material'
+  favicon: assets/logo/favicon-32x32.png
+  logo: assets/logo/dmo-logo-white.svg
   icon:
     repo: fontawesome/brands/github
   features: