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.html

@@ -84,7 +84,7 @@ along with this program. If not, see <https://www.gnu.org/licenses/>.
         <a href="https://matrix.to/#/#podpora:mxchat.cz" class="btn" target="_blank" rel="noopener">
           <i class="fa fa-matrix-org"></i> #podpora:mxchat.cz
         </a>
-        <a href="https://mamutovo.cz/@mxchatcz" class="btn" target="_blank" rel="noopener">
+        <a href="https://mamutovo.cz/@oscloud" class="btn" target="_blank" rel="noopener">
           <i class="fa fa-mastodon"></i> Mastodon
         </a>
       </nav>