added data structure docs

[joaquintides]

As discussed, added a self-contained section on our open-addressing data structure. Also, I've expanded the release notes for 1.81 with a couple of new features that went unmentioned.

[cppalliance-bot]

An automated preview of the documentation is available at https://170.unordered.prtest.cppalliance.org/libs/unordered/doc/html/unordered.html

[codecov[bot]]

Codecov Report

Merging #170 (ffa5e6f) into develop (ee8f2b9) will not change coverage. The diff coverage is n/a.

:exclamation: Current head ffa5e6f differs from pull request most recent head 46fc4f9. Consider uploading reports for the commit 46fc4f9 to get more accurate results

Additional details and impacted files

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[cppalliance-bot]

An automated preview of the documentation is available at https://170.unordered.prtest.cppalliance.org/libs/unordered/doc/html/unordered.html

[cppalliance-bot]

An automated preview of the documentation is available at https://170.unordered.prtest.cppalliance.org/libs/unordered/doc/html/unordered.html