Add external links to the "Developer Documentation" page
Mention BIP as the source for some links
Separate external links from internal links
Merge branch 'dev' into devdoclinks

Conflicts:
	en/developer-documentation.md
Additional improvements suggested by @cbeams and @harding (thanks!)
Simplify dev-doc layout with minimal text links and external link icons
Add SVG link icon
Add support for old browsers
Merge pull request #117 from saivann/devdoclinks

Add external links to the "Developer Documentation" page