fix #8419 fix #8420 ; workaround #6071 workaround nim-lang/website#98

[timotheecour]

/cc @Varriount

• fix [nimweb] ./koch web -o:/tmp/d13/ => o is actually unused and docs output dir hardcoded regardless of o #8419
• fix locally built docs point to wrong branch in Source link (Edit one is ok though) #8420
• workaround nim doc2 ignores --docSeeSrcUrl parameter #6071
• workaround [feature] access docs for devel branch website#98
• Abroken: fixes fact that locally built website points to broken urls
• new nim doc flag: --git.devel to override hardcoded devel in config/nimdoc.cfg ; useful for git repos in which the development branch is not devel (it usually is master, not devel)
• some minor code cleanups

eg usage:

./koch web --output:/tmp/d20
open /tmp/d20/index.html # see website and docs, with non-broken links

./koch web --output:/tmp/d20 --git.commit:devel
# also shows correct source links (pertain to branch devel instead of stable on website)

./koch web --output:/tmp/d20 --git.commit:myfeaturebranch --git.url:https://github.com/timotheecour/Nim
# source/edit links use provided base url, so we can see docs for un-merged PR, or un-pushed or old commit 

#NOTE: can use --git.commit:`git rev-parse --abbrev-ref HEAD` to get current branch

./koch web --output:/tmp/d20 --onlyDocs  --git.commit:devel
open /tmp/d20/docs/ospaths.html # see docs for ospaths.html (avoids building rest)

• Abroken: this PR takes care of making the local links works by setting docHTMLOutput to $output / "docs" when user provides --output
  (before that PR, locally generated docs leads to broken links when starting from web/upload, eg: file:///Users/timothee/git_clone/nim/Nim/web/upload/documentation.html has href to nonexisting file:///Users/timothee/git_clone/nim/Nim/web/upload/docs/lib.html)

for future PR:

• new nim doc flag: --git.commit_current to pass in git rev-parse --abbrev-ref HEAD as value for git.commit which is a good default when developping: we want to see docs for branch we're currently on => see [WIP] [doc] miscellaneous issues #8477
• remove references to docSeeSrcUrl in docs (instead, reference git.commit, git.devel, git.url)
  => tracked here nim doc2 ignores --docSeeSrcUrl parameter #6071