Update jdt.ui's main README

[HannesWell]

What it does

Update links, remove obsolete sections and use simpler markdown style, similar to jdt.core's README.

Author checklist

• I have thoroughly tested my changes
• The change is following the coding conventions
• I have signed the Eclipse Contributor Agreement (ECA)

[HannesWell]

With the announced deprecation of php-content on eclipse.dev websites I assume this link will not work well for a long time, but at the moment there is nothing better I know.

[jjohnstn]

I don't think you should point to the php page. It is out of date and points to other out of date pages. I would suggest you either leave the link above (which is archived) or create a new .md file by stealing any relevant text from the php page you pointed to, replacing or simply removing any php links. This at least gives a base for building on.

[HannesWell]

@jjohnstn can you please review this?

[jjohnstn]

@HannesWell @jukzi I was on vacation. Will do.

[jjohnstn]

I don't think you should point to the php page. It is out of date and points to other out of date pages. I would suggest you either leave the link above (which is archived) or create a new .md file by stealing any relevant text from the php page you pointed to, replacing or simply removing any php links. This at least gives a base for building on.