README.md: refresh and refactor

README.md: refresh and refactor

- Use &nsbp; in links with spaces
- Change headings from verbs to nouns
- Combine build and install headings
- In 'What is this', use the text on the labwc website
- Remove video links; one wasn't working and the other were very old
- Use more recent screenshot
- Simplify the 'labwc supports the following' bullet points
- Split dependencies into build and run-time
- Remove dependecy version numbers and let meson.build be the source of
  truth
- Simplify config file table by taking out middle column
- Split the configuration section into configuration and theming
- s/roadmap/scope
- Change man page links from docs/ to labwc.github.io/