feat(web): tutorials, mastodon @oscloud, GnuPG fingerprint update

- add navody/index.html as tutorial hub
- add navody/zacatek.html: complete onboarding tutorial (registration + encryption)
- 9 screenshots from chat.mxchat.cz Element web
- update Mastodon link from @mxchatcz to @oscloud (single account, OSCloud branding)
- update GnuPG fingerprint in kontakty (new YubiKey primary key 6305D75B)
- markdown artifact cleanup

index/kontakty/index.html

@@ -49,7 +49,7 @@ along with this program. If not, see <https://www.gnu.org/licenses/>.
         <a href="/index.html" class="btn">Domů</a>
       </nav>
       <nav>
-       <a href="https://mamutovo.cz/@mxchatcz" class="btn" target="_blank"
+       <a href="https://mamutovo.cz/@oscloud" class="btn" target="_blank"
           ><i class="fa fa-mastodon"></i> Mastodon</a>
         <a href="https://matrix.to/#/@archos:mxchat.cz" class="btn" target="_blank"
           ><i class="fa fa-matrix-org"></i> Matrix</a
@@ -61,7 +61,7 @@ along with this program. If not, see <https://www.gnu.org/licenses/>.
         <a href="mailto:podpora@oscloud.cz" class="btn" target="_blank">
     <i class="fa fa-envelope"></i> Email
   </a>
-  <a href="https://keys.openpgp.org/vks/v1/by-fingerprint/33E90E50DCCD1604BCEA17EA28E9AF0110058235" class="btn" target="_blank">
+  <a href="https://keys.openpgp.org/vks/v1/by-fingerprint/6305D75BFFC4ED23F3EC407370B32BB2BEAF41C9" class="btn" target="_blank">
             <i class="fa fa-key"></i> GnuPG
         </a>