docs: remove under construction notice from index

[mmolari]

Following an email exchange with Koji Yahara I modified the documentation as follows:

• I added a warning on the plasmid rearrangement tutorial, to specify that the graph must have at least one core block for the R-script to work.
• I removed the warning on top of the landing page in the documentation warning the users that pangraph is under active development. I think at this point rust-pangraph is stable enough, and the warning might discourage users from using the new version.
• I removed the migration warning on top of the readme.
• I updated docusaurs version