Improve documentation and landing page

[timfel]

From IRC:

mkoubaa> actually, Mark did talk to HPy first. See here: #323
mkoubaa> I think the mental model of "the API is mostly a struct of pointers to functions" is hard to grasp initially. However, this is exactly what a vtable looks like and most people can follow that. I think HPy can do a better job of explaining why it made that choice for HPyContext
mkoubaa> The HPy github repo does a poor job of saying "this is the definition of the HPyContext in this header file", instead relying on codegen. I don't fully understand why we don't just have a hand-crafted hpy.h that defines the API and then implementation-specific header files for universal, cpython, etc. But I also haven't made a big effort to understand the codegen system

Also Mark seemed to have a hard time finding documentation: markshannon/New-C-API-for-Python#1 (comment)

[fangerer]

I think this issue can serve as a meta issue for all documentation-related issues in the 0.9 milestone:

• Documentation of the ctx object #149
• Improve TupleBuilder docs/tests #320
• What is the actual API? #323

[fangerer]

I've also updated the landing page (i.e. hpyproject.org) but we plan to do further improvements (e.g. adding some nice benchmarks ...).