Implement documentation search

[jolheiser]

This uses a gist linked from the Hugo documentation for adding a search to your site.
I chose this method because it doesn't require any other build steps, just hugo.

It combines a few of the changes made by comments as well, such as forgoing jQuery so as to not add a dependency to it.

I added a few other languages based on indexes we have available. I admittedly just translated with Google, so feel free to correct them if it butchered them too badly.
I also moved the Help side-item to the top so it is more prominent.

I do not consider myself a UI or JS expert, so feel free to add corrections if you feel they are necessary.

[codecov-io]

Codecov Report

Merging #8937 into master will increase coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #8937      +/-   ##
==========================================
+ Coverage   41.22%   41.27%   +0.05%     
==========================================
  Files         547      546       -1     
  Lines       70764    71005     +241     
==========================================
+ Hits        29173    29308     +135     
- Misses      37870    37979     +109     
+ Partials     3721     3718       -3

Impacted Files	Coverage Δ
modules/indexer/indexer.go	44.73% <0%> (-10.53%)	⬇️
modules/git/blame.go	76.05% <0%> (-2.21%)	⬇️
services/comments/comments.go	26.31% <0%> (-1.86%)	⬇️
routers/repo/wiki.go	38.88% <0%> (-1.71%)	⬇️
modules/repofiles/action.go	64.77% <0%> (-1.51%)	⬇️
modules/log/event.go	64.61% <0%> (-1.03%)	⬇️
modules/git/repo.go	44.07% <0%> (-1.03%)	⬇️
routers/repo/compare.go	50% <0%> (-0.44%)	⬇️
services/pull/commit_status.go	8.16% <0%> (-0.35%)	⬇️
services/mirror/mirror.go	18.7% <0%> (-0.24%)	⬇️
... and 34 more

---

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 cda8de2...aa5cb88. Read the comment docs.

[jolheiser]

I should amend my original statement, as this implementation does use two libraries, fuse.js and mark.js

Currently they are included as CDN links, since our docs are online. If you'd rather I vendor them, let me know. (would they just be vendored in docs somehow??)

[mrsdizzie]

Currently docs page already uses CDN for font awesome so regardless I don't think that discussion needs to block this PR