Update the website for 4.0.0 release

[berezovskyi]

Shall we update the website for the upcoming release? At least in a pull request.

See the forum post draft for tomorrow: https://hackmd.io/QEK5pMnJQZK3sz9IT0I7Kg. There I specifically write:

• Explore the newly consolidated Lyo documentation as well as 4.0.0.RC Javadocs (now all in one place).
• Check out the OSLC 2020 Reference Implementation that uses Lyo 4.0 already! This is also a great time to take Lyo Designer for a spin and use it to study the models used to generate the majority of the reference implementation.

@jadelkhoury @jamsden will you please give me a hand with this? Feel free to edit the list below.

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TODOs

• Update oslc4j-client references with oslc-client
• Mention availability on Maven Central and move SNAPSHOT repos onto a separate page (Participating in Lyo development, for example)
• Add a 4.0 migration page. See Add migration/update guidance eclipse-lyo/lyo#15
• Include oslc-java-client to oslc-client migration guide
• Highlight that legacy oslc-java-client is ONLY for use outside a JAX-RS server running Lyo 4.0, i.e. in ETL scripts and other Java SE apps.
• Introduce RootServicesHelper Update oslc4j-client-samples to use new 4.0.0 client lyo-samples#38 (comment)
• Introduce IOslcClient Update oslc4j-client-samples to use new 4.0.0 client lyo-samples#38 (comment)

[berezovskyi]

And now we have support for editing the site right from the browser if you are too lazy to check out the repo to fix a typo (you are not the main audience for this feature, it's more for regular Lyo users):

[berezovskyi]

Also just added an ability to link to a heading directly:

[berezovskyi]

added guides for migration all the way to Lyo 6