Generalize SPEC_HEADERNAV to HEADERNAV

[wilzbach]

With the DDoc wrapper it's now pretty easy to do cool things without big Makefile hacks.
This PR now uses the Ddoc wrapper for all Ddoc pages and as a motivation starts to add HEADERNAV to a few pages.

[dlang-bot]

Thanks for your pull request, @wilzbach!

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[CyberShadow]

FYI tests failing (undefined macros).

[wilzbach]

FYI tests failing (undefined macros).

Hehe I'm so glad we added this sanity check :)
(it's passing now)
Seems like the stdin trick, leads to SRCFILENAME being defined to __stdin by default:

diff --git a/web/ascii-table.html b/web/ascii-table.html
index 5158ca7..0f8e962 100644
--- a/web/ascii-table.html
+++ b/web/ascii-table.html
@@ -89,7 +89,7 @@
 		<div >			If you spot a problem with this page, click here to create a Bugzilla issue.
 		</div>
 	</div>
-	<div class="tip smallprint">		<a href="https://github.com/dlang/dlang.org/edit/master/ascii-table.dd">Improve this page</a>
+	<div class="tip smallprint">		<a href="https://github.com/dlang/dlang.org/edit/master/__stdin.d">Improve this page</a>
 		<div >			Quickly fork, edit online, and submit a pull request for this page.
 			Requires a signed-in GitHub account. This works well for small changes.
 			If you'd like to make larger changes you may want to consider using

I added a quick commit to test how easy it would be to inject "dynamic" macros with a simple trick. I really want to avoid re-implementing Ddoc.

[wilzbach]

@CyberShadow or others: This is ready now.
I also fixed a bug I introduced when generating the spec header:

diff --git a/web/spec/property.html b/web/spec/property.html
index 19d2881..f2d8017 100644
--- a/web/spec/property.html
+++ b/web/spec/property.html
@@ -94,7 +94,7 @@
 		<div >			If you spot a problem with this page, click here to create a Bugzilla issue.
 		</div>
 	</div>
-	<div class="tip smallprint">		<a href="https://github.com/dlang/dlang.org/edit/master/__stdin.d">Improve this page</a>
+	<div class="tip smallprint">		<a href="https://github.com/dlang/dlang.org/edit/master/spec/property.dd">Improve this page</a>
 		<div >			Quickly fork, edit online, and submit a pull request for this page.
 			Requires a signed-in GitHub account. This works well for small changes.
 			If you'd like to make larger changes you may want to consider using

[wilzbach]

So this fixes the "Improve this page" link for all spec pages and - even though it changes a couple of pages - it is pretty trivial. However, if the number if changes is a problem I can also split them of into a separate PR.

[wilzbach]

Rebasing this gets tedious - any reason for not moving forward with this?

[wilzbach]

(another rebase)