Add a Fossil feature

Dependencies

• [2] 73PQOETU Update slides to 2021 situation
• [*] ARAFRHKU Create the outline of the talk
• [*] UXLO2KVD Introduce fragments
• [*] 5LKTNB6O LOTS of progress

Change contents

• edit in sections/5-fossil.md at line 24
  [5.3768]

  [5.3768]
  + <li class="fragment fade-in-then-semi-out">has a 'preserve all history' philosophy</li>
• edit in sections/5-fossil.md at line 34
  [2.56620]

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  + * **preserve all history** - there is no rebase in Fossil. Also: why would it be called 'Fossil'?