doc: Added a document listing all vendor extensions of Vaev.

src/web/doc/vendor.md

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+# List of Vendor Extensions
+
+**Disclaimer**: Theses extensions are not part of any standard and will change without notice. Don't rely on them. This document is for informational purposes only.
+
+ - @-vaev-top : Select the box containing the top left, center, and right margins of the page.
+ - @-vaev-bottom : Select the box containing the bottom left, center, and right margins of the page.
+ - @-vaev-left : Select the box containing the left top, center, and bottom margins of the page.
+ - @-vaev-right : Select the box containing the right top, center, and bottom margins of the page.
+ - var(-vaev-datetime) : Date and time of the printout.
+ - var(-vaev-title): Title of the document.
+ - var(-vaev-url): URL of the document.