Upgrade to Dokka 1.4.20

[sdeleuze]

See https://blog.jetbrains.com/kotlin/2020/08/dokka-preview-based-on-kotlin-1-4-0-rc/, depends on Kotlin/dokka#1293.

[sdeleuze]

I have made progresses (see this WIP commit on a branch) but I am currently blocked by Kotlin/dokka#1466 that I raised as a question. Let's wait Dokka team answer.

[sdeleuze]

Running ./gradlew dokkaHtmlMultiModule now succeeds but there are 2 blocking remaining issues:

• A README.md is required in each modules, this requirement will be relaxed in Dokka 1.4.10 to be released shortly
• The resulting root HTML file is empty
  

So I move it to 5.3 RC2 to try again with Dokka 1.4.10.

@bclozel Could you please have a look to the draft commit in order to say me if you prefer a dedicated gradle/docs-subprojects.gradle file with inclusion or what I did?

[sdeleuze]

I tried with Dokka 1.4.10 but we need more improvement on multi module handling in order to use it, see Kotlin/dokka#1302 (comment).

[jhoeller]

Addressed as part of #26870.