Add doc links to main doc and formatting

[nw0]

Hi, occasional user of this crate here! I've seen #327 and found the book reasonably helpful, but it was not at all obvious to me that the book existed at first.

This PR adds a link to the book at the top of the crate's rustdoc, as well as some intra-doc links for the formatting items.

[codecov[bot]]

Codecov Report

Merging #554 (abce6e0) into main (d7c725c) will decrease coverage by 0.0%. The diff coverage is n/a.

@@           Coverage Diff           @@
##            main    #554     +/-   ##
=======================================
- Coverage   95.7%   95.7%   -0.0%     
=======================================
  Files         78      78             
  Lines       8695    8703      +8     
=======================================
+ Hits        8325    8330      +5     
- Misses       370     373      +3     

Impacted Files	Coverage Δ
time/src/formatting/formattable.rs	100.0% <ø> (ø)
time-macros/src/serde_format_description.rs	93.9% <0.0%> (-1.9%)	:arrow_down:
time/src/serde/mod.rs	96.5% <0.0%> (ø)
.../format_description/well_known/iso8601/adt_hack.rs	100.0% <0.0%> (ø)
time-macros/src/lib.rs	72.7% <0.0%> (+0.6%)	:arrow_up:

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