Various READMEs and docs cleanup

Noticeably closes #11428.
Trim src/ README, bring back version_info everywhere
Tighten up float literal lexing.

Specifically, dissallow setting the number base for every type of float
literal, not only those that contain the decimal point. This is in line with
the description in the manual.
Add /doc/rustuv to .gitignore
auto merge of #11477 : adridu59/rust/bug-report, r=cmr

Mostly cleanups for doc and READMEs. Fixes the bug reporting link.
auto merge of #11480 : SiegeLord/rust/float_base, r=cmr

This fixes the incorrect lexing of things like:

~~~rust
let b = 0o2f32;
let d = 0o4e6;
let f = 0o6e6f32;
~~~

and brings the float literal lexer in line with the description of the float literals in the manual.
auto merge of #11483 : kballard/rust/gitignore-doc-rustuv, r=alexcrichton