Use generic link to Dulwich website

[Morganov]

• [x] I provide my work under the project license.
• [x] I grant such license of my work as is required for the purposes of future print editions to Ben Straub and Scott Chacon.

Changes

• Due to errors on loading particular webpages from dulwich.io the HTML checker fails on each build. And it's better to use the main page only instead of the list of pages.

[ben]

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