Update mdbook configuration

This commits explicitly sets many configuration options for the mdbook generation.
In particular, for generic options (see https://rust-lang.github.io/mdBook/format/configuration/general.html#build-options):
- 'create-missing' is set to false. This will make the build process exit with an error if any files do not exist in 'SUMMARY.md'.
- 'use-default-preprocessors' is set to false. This disables the default preprocessors ('links' & 'index').
The 'index' preprocessor converts all chapter files named 'README.md' into 'index.md', but there is a catch: relative links are not dynamically updated (see rust-lang/mdBook#1921), leading to errors at build generation. We can live just fine without that plugin.
The 'links' preprocessor expands the {{ #playground }}, {{ #include }}, and {{ #rustdoc_include }} handlebars helpers in a chapter to include the contents of a file. We can probably live without too (although it could be re-enabled).

And for specific "linkcheck" options (see https://github.com/Michael-F-Bryan/mdbook-linkcheck/blob/v0.7.7/README.md?plain=1#L64-L125):
- 'optional' is set to true. This will output a warning if the plugin is not installed, rather than failing to build entirely.
- 'warning-policy is set to 'warn'. This changes the level of messages to warnings, rather than errors that interrupt the build process.
- 'traverse-parent-directories' is set to false. This disallows linking to files outside of the book's root directory.
- 'exclude' is set to some links. This excludes the linkchecking from running on those. They were chosen after trial and error.

Author: tgreenx

docs/README-for-doc.zonemaster.net.md

@@ -49,5 +49,5 @@ The public documentation is divided into the following main categories:
 [The Swedish Internet Foundation]:                  https://internetstiftelsen.se/en/
 [Zonemaster.net]:                                   https://zonemaster.net
 
-<!-- Location on doc.zonemaster.net: --> 
+<!-- Location on doc.zonemaster.net: -->
 [Zonemaster-logo]:                                  ../zonemaster_logo_2021_color.png

docs/public/book.toml

@@ -4,14 +4,20 @@ description = "Zonemaster documentation"
 language = "en"
 src = "./"
 
+[build]
+create-missing = false
+use-default-preprocessors = false
+
 [output.html]
 no-section-label = true
 
 [output.html.fold]
 enable = true
 level = 1
 
-[preprocessor.index]
-
 [output.linkcheck]
+optional = true
+warning-policy= "warn"
 follow-web-links = false
+traverse-parent-directories = false
+exclude = [ 'SUMMARY.md', 'LICENSE.md', 'zonemaster_logo_2021_color.png', 'localhost' ]