[jampukka]

Refreshes the readme, which had drifted from the code.

• Drops the community module table. It was out of date (it lacked bytes and the tiles modules), it took up a big part of the readme although community modules are not central to the project, and docs/overview/modules.md and CONTRIBUTING.md already cover them
• Intro: fixes the module that builds the webapp (hakunapi-simple-webapp-jakarta, not hakunapi-simple-servlet), lists the output formats, states the focus as open data stores such as PostGIS and GeoPackage, and explains the core/community split in two sentences with a link to the module docs
• New Documentation and Building sections (Java 21, mvn clean install, where features.war ends up); the examples section lists both examples
• Docker notes: the Tomcat image is tomcat:jdk21-openjdk (not pinned), and the hot reload image installs JBR 21 for DCEVM; typos fixed