Add support to use mdoc

[rafaparadela]

This PR closes #292

What it does basically is:

• Adds a new sbt setting micrositeCompilingDocsTool to chose between tut and mdoc.
• Edits the task makeMicrosite to run either makeTut or makeMdoc according to the value of micrositeCompilingDocsTool.
• Adds the sbt-mdoc pluging.
• Adds a scripted test to prove the compilation of mdoc snippets
• Adds a scripted test to prove how makeMicrosite ends without effects when the micrositeCompilingDocsTool has an invalid value.
• Updates the documentation to explain how operate with both options and a detailed guide for migrating from tut to mdoc.

[rafaparadela]

@juanpedromoreno It seems that Travis gives us a red in the 0.13 & 2.10 version as we discussed yesterday.

[juanpedromoreno]

@rafaparadela I'll try to have a look at this today. Thanks.

[calvellido]

This is great Rafa 👏👏

[juanpedromoreno]

The code looks good to me @rafaparadela , thank you!!

[MaureenElsberry]

@rafaparadela done reviewing the docs, only made tiny changes. Thanks!

[rafaparadela]

Thanks a lot @MaureenElsberry