Re-order GPG signing docs and fix code blocks

[jolheiser]

Moved the section about generating a GPG key to before the config options, as it is technically a pre-requisite and also re-enforces that Gitea will use a specific key for signing rather than a user's etc.

Also changed some code blocks that currently don't render correctly.

[codecov-io]

Codecov Report

Merging #10349 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #10349      +/-   ##
==========================================
+ Coverage   43.69%   43.71%   +0.02%     
==========================================
  Files         585      585              
  Lines       81019    81019              
==========================================
+ Hits        35398    35419      +21     
+ Misses      41230    41214      -16     
+ Partials     4391     4386       -5

Impacted Files	Coverage Δ
modules/process/manager.go	74.69% <0%> (-3.62%)	⬇️
modules/queue/workerpool.go	56.93% <0%> (-1.07%)	⬇️
services/pull/pull.go	33.93% <0%> (+1.5%)	⬆️
modules/queue/unique_queue_disk_channel.go	55.76% <0%> (+1.92%)	⬆️
services/pull/patch.go	62.89% <0%> (+2.51%)	⬆️
services/pull/temp_repo.go	31.62% <0%> (+2.56%)	⬆️
modules/git/command.go	89.56% <0%> (+2.6%)	⬆️
services/pull/check.go	38.41% <0%> (+5.48%)	⬆️

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