doc: doap: Add RFCs and sort elements consistently

[melvo]

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: 68.61% // Head: 68.61% // Decreases project coverage by -0.00% :warning:

Coverage data is based on head (21fd3d9) compared to base (6156452). Patch has no changes to coverable lines.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #474 +/- ## ========================================== - Coverage 68.61% 68.61% -0.01% ========================================== Files 283 283 Lines 24573 24572 -1 ========================================== - Hits 16860 16859 -1 Misses 7713 7713 ``` | [Impacted Files](https://codecov.io/gh/qxmpp-project/qxmpp/pull/474?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qxmpp-project) | Coverage Δ | | |---|---|---| | [tests/qxmppomemodata/tst\_qxmppomemodata.cpp](https://codecov.io/gh/qxmpp-project/qxmpp/pull/474/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qxmpp-project#diff-dGVzdHMvcXhtcHBvbWVtb2RhdGEvdHN0X3F4bXBwb21lbW9kYXRhLmNwcA==) | `98.24% <0.00%> (-0.01%)` | :arrow_down: | 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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