manual: clarify VSpace as top-level paging structure

This change updates the vspace chapter to separate the high-level concept of a VSpace from the
architectually defined objects. It also updates the various names for the vspace parameter to all be
vspace.

manual: refactor vspace chapter

- instead of referring to only PDs and PTs, use generic language about paging structures
- link to each architectures methods
- create tables that show the info for all archictures

manual: remove hard-coded PD, PT and Page invocations

- increases the maintainability of the docs
- move descriptions of methods to the API reference
- remove hardcoded invocations in manual

This commit removes the content completely from vspace.tex, as it is out of date and needs
restructuring. The next commit does the restructuring.

manual: remove hard-coded ASID invocations

- move documentation to API ref
- create tables pointing to the API ref for these objects

riscv: document page table structure

Manual: vspace.tex - document cache operations on page directories

manual: Correct ia32 to x86 in some sections

SELFOUR-254: manual: describe mapping attributes

Correctly describes the fact that on ARM mapping attributes are
a set of flags that can be OR'd together, and on x86 they are an
enumeration of possible values

manual: Purely cosmetic changes / improved wording

further support for XN bit on ARM

Release snapshot