Improve and update OMEMO documentation

[melvo]

• OmemoManager: Use new CarbonManagerV2 and addNewExtension() for class documentation
• OmemoManager: Remove unneeded Doxygen references for method documentation
• doc: index: Add OmemoManager

PR check list:

• [x] Document your code
• [x] Add \since QXmpp 1.X, QXMPP_EXPORT
• [x] Fix doxygen warnings (see log when building with -DBUILD_DOCUMENTATION=ON)
• [x] Update doc/doap.xml
• [x] Add unit tests
• [x] Format the code: Run clang-format -i src/<edited-file(s)> tests/<edited-file(s)>

[codecov[bot]]

Codecov Report

Base: 68.36% // Head: 68.36% // Increases project coverage by +0.00% :tada:

Coverage data is based on head (82aac42) compared to base (befab2f). Patch has no changes to coverable lines.

:exclamation: Current head 82aac42 differs from pull request most recent head 1a2f8ff. Consider uploading reports for the commit 1a2f8ff to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #514 +/- ## ======================================= Coverage 68.36% 68.36% ======================================= Files 298 298 Lines 25869 25870 +1 ======================================= + Hits 17685 17686 +1 Misses 8184 8184 ``` | [Impacted Files](https://codecov.io/gh/qxmpp-project/qxmpp/pull/514?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qxmpp-project) | Coverage Δ | | |---|---|---| | [src/omemo/QXmppOmemoManager.cpp](https://codecov.io/gh/qxmpp-project/qxmpp/pull/514/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qxmpp-project#diff-c3JjL29tZW1vL1FYbXBwT21lbW9NYW5hZ2VyLmNwcA==) | `16.22% <ø> (ø)` | | | [tests/qxmppomemodata/tst\_qxmppomemodata.cpp](https://codecov.io/gh/qxmpp-project/qxmpp/pull/514/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qxmpp-project#diff-dGVzdHMvcXhtcHBvbWVtb2RhdGEvdHN0X3F4bXBwb21lbW9kYXRhLmNwcA==) | `98.25% <0.00%> (+<0.01%)` | :arrow_up: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qxmpp-project). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qxmpp-project)

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