- Update the main README to reflect the current project status
- Rework the main API generation documentation. General fixes/tidying,
but also spell out explicitly how to make API changes at the top of the
document since this is what most people will care about.

---------

Co-authored-by: Martin Grant <martingrant@outlook.com>

This removes the `ol_impl_result_t` helper class, replacing it with
`llvm::Error`. In addition, some internal functions that returned
`ol_errc_t` now return `llvm::Error` (with a fancy message).

A new ErrorCode enumeration is present in PluginInterface which can
be used when returning an llvm::Error from offload and PluginInterface
functions.

This enum must be kept up to sync with liboffload's ol_errc_t enum, so
both are automatically generated from liboffload's enum definition.

Some error codes have also been shuffled around to allow for future
work. Note that this patch only adds the machinery; actual error codes
will be added in a future patch.

~~Depends on #137339 , please ignore first commit of this MR.~~ This has
been merged.