Migrate plone.exportimport documentation from its docs to its README.md and to Plone 6 Documenation

[stevepiercy]

Important

Merge related PR first: plone/documentation#1842

Closes #46

[mister-roboto]

@stevepiercy thanks for creating this Pull Request and helping to improve Plone!

TL;DR: Finish pushing changes, pass all other checks, then paste a comment:

@jenkins-plone-org please run jobs

To ensure that these changes do not break other parts of Plone, the Plone test suite matrix needs to pass, but it takes 30-60 min. Other CI checks are usually much faster and the Plone Jenkins resources are limited, so when done pushing changes and all other checks pass either start all Jenkins PR jobs yourself, or simply add the comment above in this PR to start all the jobs automatically.

Happy hacking!

[stevepiercy]

@jenkins-plone-org please run jobs

[stevepiercy]

Something unrelated to this PR broke the Jenkins build:

https://jenkins.plone.org/job/pull-request-6.1-3.13/60/testReport/plone.app.multilingual.tests.test_helper_views/PAMIntTestHelperViews/test_move_content_existing_translation_inside/

[mauritsvanrees]

Mostly fine, thanks!
Some change requests.

[mauritsvanrees]

Something unrelated to this PR broke the Jenkins build:

Indeed unrelated.

Actually it is useless to run Jenkins jobs for this repository: its tests are not found. The reason is the same as in this issue for plone.distribution.

We could configure mr.roboto to not add its remark. But maybe we will fix the issue in the near future.

[stevepiercy]

I'll forego running Jenkins again, as it is pointless.

[mauritsvanrees]

Thanks!