OC-5 [www] Prepare documentation site for 4.2

[marek-parfianowicz]

Take exports of Confluence pages from atlassian-docs.bitbucket.io and copy them to openclover.org.

Update this documentation for OpenClover 4.2 release content:

merge user manual and knowledge base spaces into one /done/
apply CSS styling /done/
add navigation header /done/
add copyright footer /done/
replace com.atlassian.* artifactId by openclover ones, update code snippets /done/
use 'OpenClover 4.2' title instead of 'Clover 4.1' /done/
hook documentation under the openclover.org/documentation, redesign the main page /done/
remove obsolete attachments /done/
remove obsolete pages /done/

[marek-parfianowicz]

Original report: https://bitbucket.org/openclover/clover/issues/5