Update the README file to follow the manual introduction closer.

It makes sense for the introduction to be a longer version of the README, not
a completely different topic.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75172 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Fix the 3.0.0 release date and update the release announcement slightly.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75173 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Update the version in wx/version.h and add an item for it to the docs.

Mention the need to udpate this file in the release checklist.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75174 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Remove "rc2" from the release notes.

The hashes are still the old ones and will be updated later as the binaries
are not ready yet.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75175 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775