Updates theme names to be compatible with Godot 3.5 (no spaces),
consistent with other Godot theme item names (snake_case), and
match the color names listed on the
[ANSI escape code wikipedia page](https://en.wikipedia.org/wiki/ANSI_escape_code#3-bit_and_4-bit).
Deprecates the old names and warns users to change them.
As cols and rows are read only (i.e. automatically determined by rect
and font size) there is no need for the properties to be exposed.
Instead, users can get the calculated cols and rows using the get_cols()
and get_rows() methods.
Makes it easier to keep documentation up to date and can deploy with
GitHub pages.
README.md moved to docs/README.md and is shared with GitHub Pages home
page.
