[docs] Use relative URLs for man pages

[aaronpuchert]

Summary

Should have no effect on the online documentation, but it makes offline
builds more self-contained. With relative links however we have to
abstain from using :manpage: outside of man page cross-references.

llvm/docs/SymbolizerMarkupFormat.rst

 
 This format identifies markup elements with a syntax that is both simple and
 distinctive. It's simple enough to be matched and parsed with straightforward
 code. It's distinctive enough that character sequences that look like the start
 or end of a markup element should rarely if ever appear incidentally in logging
 text. It's specifically intended not to require sanitizing plain text, such as
 the HTML/XML requirement to replace ``<`` with ``&lt;`` and the like.
 
-:manpage:`llvm-symbolizer(1)` includes a symbolizing filter via its ``--filter``
-option.
+:doc:`llvm-symbolizer <CommandGuide/llvm-symbolizer>` includes a symbolizing
+filter via its ``--filter`` option.
 
 Scope and assumptions
 =====================
 
 A symbolizing filter implementation will be independent both of the target
 operating system and machine architecture where the logs are generated and of
 the host operating system and machine architecture where the filter runs.
 
``{{{mmap:%p:%i:...}}}``
 
   Example::
 
     {{{mmap:0x7acba69d5000:0x5a000:load:1:rx:0x1000}}}
 
 .. rubric:: Footnotes
 
 .. [#not_yet_implemented] This markup element is not yet implemented in
-  :manpage:`llvm-symbolizer(1)`.
+  :doc:`llvm-symbolizer <CommandGuide/llvm-symbolizer>`.

llvm/docs/conf.py

 man_pages = []
 
 # Automatically derive the list of man pages from the contents of the command
 # guide subdirectory.
 basedir = os.path.dirname(__file__)
 man_page_authors = "Maintained by the LLVM Team (https://llvm.org/)."
 command_guide_subpath = 'CommandGuide'
 command_guide_path = os.path.join(basedir, command_guide_subpath)
+manpages_url = '{page}.html'
 
 
 def process_md(name):
     file_subpath = os.path.join(command_guide_subpath, name)
     with open(os.path.join(command_guide_path, name)) as f:
         title = f.readline().rstrip('\n')
 
         m = re.match(r'^# (\S+) - (.+)$', title)

 #man_show_urls = False
 
 # FIXME: Define intersphinx configuration.
 intersphinx_mapping = {}
 
 # Pygment lexer are sometimes out of date (when parsing LLVM for example) or
 # wrong. Suppress the warning so the build doesn't abort.
 suppress_warnings = [ 'misc.highlighting_failure' ]
-
-# Direct html-ified man pages to llvm.org
-manpages_url = 'https://llvm.org/docs/CommandGuide/{page}.html'