Updated readme with links to learn path and docs.microsoft.com

[StefanIvemo]

Had a look at the Get started with Bicep section in the readme after answering to discussion #4035.

Changes:

• Replaced the link to the Bicep Tutorial in the repo with a link to the learn module.
• Updated links that pointed to the docs in the repo with new links to docs.microsoft.com

Contributing to documentation

• The contribution does not exist in any of the docs in either the root of the docs directory or the specs

[alex-frankel]

good changes - thanks!