docs: generate documentation for non-public symbols

[vitorhugods]

---

PR Submission Checklist for internal contributors

• The PR Title

  • conforms to the style of semantic commits messages¹ supported in Wire's Github Workflow²
  • answers the question: If merged, this PR will: ... ³

• The PR Description

  • is free of optional paragraphs and you have filled the relevant parts to the best of your ability

---

What's new in this PR?

Issues

Dokka is only generating documentation for public stuff.
A lot of the documentation we do is useful for us developing it too. Especially the internal symbols.

So important classes and definitions, like EventGatherer do not have generated documentation.

Solutions

Ideally, we could specify the visibility-level of the things we want to genereate documentation from/to.
But for this, we would need Dokka 1.6.20 (see Kotlin/dokka#434 (comment)) and we're not there yet.
In the meanwhile, we can use includeNonPublic and generate for everything.

Testing

N/A

---

PR Post Merge Checklist for internal contributors

• If any soft of configuration variable was introduced by this PR, it has been added to the relevant documents and the CI jobs have been updated.

---

References

1. https://sparkbox.com/foundry/semantic_commit_messages
2. https://github.com/wireapp/.github#usage
3. E.g. feat(conversation-list): Sort conversations by most emojis in the title #SQPIT-764.

[github-actions]

Unit Test Results

   143 files   -   15     143 suites   - 15   53s ⏱️ +38s
   697 tests +    3     690 ✔️ +    2  7 💤 +1  0 ❌ ±0 
1 200 runs  +506  1 193 ✔️ +505  7 💤 +1  0 ❌ ±0 

Results for commit 561540e. ± Comparison against base commit 50b2ffc.

♻️ This comment has been updated with latest results.