docs(install): add new install method via nix

[sambacha]

What I did

• Update install directions to include directions for nix
• Fix minor spelling errors as well.
• Updated pip install method to reflect versioning of more recent compiler versions.

How I did it

Edited the page.

How to verify it

I installed vyper 0.2.11 via nix on my laptop and compiled a contract successfully.

Description for the changelog

• Documentation update: Install Vyper via Nix now in documentation.

Cute Animal Picture

No

[codecov-commenter]

Codecov Report

Merging #2405 (1331759) into master (0a389e3) will decrease coverage by 0.24%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2405      +/-   ##
==========================================
- Coverage   85.73%   85.48%   -0.25%     
==========================================
  Files          91       91              
  Lines        9026     9030       +4     
  Branches     2154     2152       -2     
==========================================
- Hits         7738     7719      -19     
- Misses        791      806      +15     
- Partials      497      505       +8     

Impacted Files	Coverage Δ
vyper/old_codegen/events.py	95.00% <0.00%> (-4.22%)	⬇️
vyper/old_codegen/context.py	91.01% <0.00%> (-2.25%)	⬇️
vyper/ast/signatures/function_signature.py	86.15% <0.00%> (-1.54%)	⬇️
vyper/semantics/types/indexable/sequence.py	84.50% <0.00%> (-1.41%)	⬇️
vyper/old_codegen/parser_utils.py	77.89% <0.00%> (-0.84%)	⬇️
vyper/old_codegen/keccak256_helper.py	78.57% <0.00%> (-0.38%)	⬇️
vyper/old_codegen/expr.py	80.11% <0.00%> (-0.38%)	⬇️
vyper/old_codegen/stmt.py	84.15% <0.00%> (-0.36%)	⬇️
vyper/semantics/validation/local.py	89.92% <0.00%> (-0.08%)	⬇️
vyper/ast/folding.py	93.61% <0.00%> (ø)
... and 13 more

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[charles-cooper]

@sambacha thanks, looking pretty good, I left some comments - main thing is I think we can skip everything that is covered by the nixos package search since we are already linking to it.

[charles-cooper]

Thanks!! LGTM