docs: fix links to AsciiDoc

Links were aimed at the methods.co.nz domain, which is now returning
404s. The current situation of AsciiDoc is unclear to me: the Fedora
package points to this website, they own asciidoc.org, Wikipedia points
to this project as well but their Git repo's README includes the
following paragraph:

> AsciiDoc.py is a legacy processor for this syntax, handling an older
> rendition of AsciiDoc. As such, this will not properly handle the
> current AsciiDoc specification. It is suggested that unless you
> specifically require the AsciiDoc.py toolchain, you should find a
> processor that handles the modern AsciiDoc syntax.

https://github.com/asciidoc-py/asciidoc-py/blob/10.1.3/README.md

"AsciiDoc specification" pointing towards:
https://projects.eclipse.org/projects/asciidoc.asciidoc-lang

Signed-off-by: Théo Lebrun <theo.lebrun@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

docs/conf/asciidoc-text.conf

@@ -1,5 +1,5 @@
 # Refer to following asciidoc documentation:
-#     http://www.methods.co.nz/asciidoc/userguide.html
+#     https://asciidoc-py.github.io/userguide.html
 # In particular sections "Macros" and "Attribute References"
 #
 # For hyperlinks, show 'link text [URL]' (if link text provided) or 'URL'

docs/manual/writing-rules.txt

@@ -207,10 +207,10 @@ https://github.com/pengutronix/genimage/blob/master/README.rst[].
 === The documentation
 
 The documentation uses the
-http://www.methods.co.nz/asciidoc/[asciidoc] format.
+https://asciidoc-py.github.io/[asciidoc] format.
 
 For further details about the asciidoc syntax, refer to
-http://www.methods.co.nz/asciidoc/userguide.html[].
+https://asciidoc-py.github.io/userguide.html[].
 
 === Support scripts