feat(book): updated book for 0.3 changes

[baszalmstra]

Changes to the book for v0.3

[codecov]

Codecov Report

Merging #314 (cf86e70) into master (aaa677a) will not change coverage.
The diff coverage is n/a.

❗ Current head cf86e70 differs from pull request most recent head 7535b32. Consider uploading reports for the commit 7535b32 to get more accurate results

@@           Coverage Diff           @@
##           master     #314   +/-   ##
=======================================
  Coverage   80.59%   80.59%           
=======================================
  Files         259      259           
  Lines       15561    15561           
=======================================
  Hits        12541    12541           
  Misses       3020     3020           

Impacted Files	Coverage Δ
crates/mun_memory/tests/diff/util.rs	83.63% <0.00%> (-0.91%)	⬇️
crates/mun_hir/src/diagnostics.rs	64.83% <0.00%> (+0.42%)	⬆️

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