Add coverall badges

Doxygen currently does not support images with links, which breaks the
current rendering of the front page at https://miloyip.github.io/rapidjson.

Move the status badges to a table with the links in the top-row instead.

While at it, add a description below the logo and put the GitHub and
documentation links into in itemisation.

Fix typesetting

So the example link in documentation will always point to correct place.

Some links will become broken in GitHub.

To avoid the {#mainpage} tag to be rendered on GitHub, explicitly add a
`\mainpage` section to `rapidjson.h`, referring to `readme.md`.

rapidjson -> RapidJSON

Can write d.Parse(...) instead of d.Parse<0>(...)
Hope to reduce strangeness and confusion for beginner.

Showing the type Value and preventing member lookup twice.

Fixes image path

minor mistake

Added a paragraph about the unicode features

Add compatibility section and minor changes about JSON.