Hi @aleksjablo,
You can read the Mattermost documentation to learn more about incoming webhooks, outgoing webhooks, and slash commands.
Mattermost uses the same format for webhooks as Slack does, so webhooks that work with Slack should also work with Mattermost. There are also a variety of integrations that have been created by the community (including ones for GitHub and Email).
LDAP is available, but requires an Enterprise license. You can learn more about Enterprise Edition and request a trial here.
There is currently no whiteboard feature in Mattermost, but you can add a feature request for it and it may be added if it's a popular idea with the community.
Finally, in Mattermost hashtags are used to tag a post with a topic (simiilar to Twitter), and they are public for everyone. However, if you only use the hashtag in a private channel, then only the people with access to the private channel will see the hashtag.
I hope that information helps