[Doc] Remove Pulsar Dashboard contents

[Anonymitaet]

Fix #11284

[Jennifer88huang-zz]

@sijia-w could you please help work on this issue?

[sijia-w]

@Anonymitaet @jennifer88huang I have checked the docs that contain Pulsar dashboard and here is what I think:

• In file administration dashboard: remove everything about the usage of dashboard and leave only a note.
• In other files that contain Pulsar Dashboard: keep the content and make sure there is a note to remind the users that we encourage them to use Pulsar Manager.

How do you think?

[Anonymitaet]

@sijia-w

#1
I think we can not remove the content in administration-dashborad.md because other .md files (old version doc) might reference some content in administration-dashborad.md. Besides, there is a note in administration-dashborad.md which already reminds and guides users to the right place.

#2
Agree with you on that.

@eolivelli @jennifer88huang thoughts?

[Jennifer88huang-zz]

@Anonymitaet agree.
The current Pulsar dashboard is not displayed on Pulsar website; yet if you search, you can find it.

[sijia-w]

@jennifer88huang @Anonymitaet Thanks for your feedback. Please help review the pr linked.