FDP primer: quick start: fix misdirections

[grahamperrin]

Summary

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=274072 was reported and taken by me, then taken away and closed twice with commits that did not fix the bug.

Fixes: 0e60961e5a28 FDP-overview: Fix build output path
Fixes: a3e71e854d22 FDP-Overview: Really fix the documentation path

Test Plan

Discussed in Matrix, 2023-09-26:

% cd
% git clone https://git.FreeBSD.org/doc.git ~/doc
Cloning into '/home/grahamperrin/doc'...
remote: Enumerating objects: 500427, done.
remote: Counting objects: 100% (10854/10854), done.
remote: Compressing objects: 100% (614/614), done.
remote: Total 500427 (delta 10730), reused 10240 (delta 10240), pack-reused 489573
Receiving objects: 100% (500427/500427), 310.92 MiB | 6.40 MiB/s, done.
Resolving deltas: 100% (345475/345475), done.
Updating files: 100% (10461/10461), done.
% cd doc/documentation && make DOC_LANG=en
---------------------------------------------------------------
Building the documentation
included languages: en
excluded languages: bn-bd da de el es fr hu id it ja ko mn nl pl pt-br ru tr zh-cn zh-tw
---------------------------------------------------------------
/usr/local/bin/ruby ./tools/global-pgpkeys-creator.rb
HUGO_DISABLELANGUAGES=" bn-bd da de el es fr hu id it ja ko mn nl pl pt-br ru tr zh-cn zh-tw" /usr/local/bin/hugo --minify
WARN  DEPRECATED: Kind "taxonomyterm" used in disableKinds is deprecated, use "taxonomy" instead.
WARN  Unknown kind "list" in outputs configuration.
WARN  Unknown kind "single" in outputs configuration.
Start building sites … 
hugo v0.118.2+extended freebsd/amd64 BuildDate=2023-09-02T09:28:48Z+0000 VendorInfo=freebsd


                   |  EN   
-------------------+-------
  Pages            | 1177  
  Paginator pages  |    0  
  Non-page files   |    0  
  Static files     |  825  
  Processed images |    0  
  Aliases          |    1  
  Sitemaps         |    2  
  Cleaned          |    0  

Total in 96722 ms
% pwd
/usr/home/grahamperrin/doc/documentation
% file ~/doc/public
/home/grahamperrin/doc/public: cannot open `/home/grahamperrin/doc/public' (No such file or directory)
% file ~/doc/documentation/public
/home/grahamperrin/doc/documentation/public: directory
%

At the foot, please note the results of the two file commands:

• ~/doc/public (part of https://github.com/freebsd/freebsd-doc/commit/a3e71e854d225ecc626c3210fc115f96c3202f18#diff-b84dd44f86b2b857e45c34fe1d19516bbf0c8d9df79a5f996881e25c236dc764R146) does not exist
• ~/doc/documentation/public does exist.