Suggestive patch for simplified completion

NOTE: The patch is largely untested.

Modifications:

- Update readme.org
- Remove geiser-company
- Move Company extensions to geiser-completion

Omissions:

- geiser-company--inhibit-autodoc has been removed. Eldoc handling
  should be implemented in the frontend, not in the backend.
  See for example:
  https://github.com/minad/corfu/blob/04fbfce3d7e9c125a7fd22a34455a508247a522b/corfu.el#L1212
- The quickhelp-string action and geiser-company--docstring have been
  removed. company-quickhelp can use `:company-doc-buffer` instead with
  minimal overhead.
  See:
  https://github.com/company-mode/company-quickhelp/blob/3ca2708b4e5190205aca01d65fe1b391963a53f9/company-quickhelp.el#L138
- The automatic Company setup has been removed. Personally I am not a
  fan of such auto configuration. It is better if completion is
  configured consistently in the user configuration. You may want to
  restore the auto configuration for backward compatibility. It depends
  on your backward compatibility story. I am fine with rare breaking
  changes from time to time.
- There is a cyclic dependency between geiser-edit/geiser-doc and
  geiser-completion, which should be untangled.