Fix documentation homepage

[ali-ramadhan]

By default browsers take you to index.html but in our case that's the function index so I've renamed home.md to index.md so it becomes the home page for documentation. The function index is now function_index.md.

[codecov]

Codecov Report

Merging #584 into master will increase coverage by 0.65%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #584      +/-   ##
==========================================
+ Coverage   72.17%   72.82%   +0.65%     
==========================================
  Files          70       70              
  Lines        2016     2050      +34     
==========================================
+ Hits         1455     1493      +38     
+ Misses        561      557       -4

Impacted Files	Coverage Δ
src/boundary_conditions.jl	62.5% <0%> (+1.44%)	⬆️
src/logger.jl	59.25% <0%> (+59.25%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5888900...b7cae54. Read the comment docs.