Documentation follow-up for the massive upgrade

[jmsche]

See PR #89 for reference.

Some things have to be changed in the README:

• Install guide:
  • using Flex
  • not using Flex, including the following details:
    • routing configuration
    • service configuration, with template
• Symfony version: 4.3+
• Link to previous docs, according to the following decision that's about to happen in #89 about branches & tags
• Remove references to jQuery

I'll update the README once the decision about branches/tags will be taken.

@sguionni Maybe you see more stuff to add to docs?

[jmsche]

Fixed via #94; needs a little update in v2 branch as well

[jmsche]

v2 docs will be fixed by #99