Add Roadmap to documentation

[philippjfr]

@holoviz-developers here is the roadmap draft based on what I presented at the last steering committee meeting.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.61%. Comparing base (85de78b) to head (909161c).
Report is 566 commits behind head on main.

❗ Current head 909161c differs from pull request most recent head 8c4c4e2. Consider uploading reports for the commit 8c4c4e2 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5443      +/-   ##
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+ Coverage   83.27%   83.61%   +0.34%     
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  Files         274      276       +2     
  Lines       40051    40152     +101     
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+ Hits        33352    33575     +223     
+ Misses       6699     6577     -122     

Flag	Coverage Δ
ui-tests	41.37% <ø> (+0.65%)	⬆️
unitexamples-tests	72.56% <ø> (-0.03%)	⬇️

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[ahuang11]

Looks good to me. Just wondering whether the ordering of these items corresponds to its priority.

[MarcSkovMadsen]

For me the most important for Panel is to reduce the friction of getting started with and using the framework. To reduce the friction I believe we need.

• Excellent documentation. We are on the right path. But for example we lack tutorials. And the quality of our old documentation needs to be improved, and to be updated to Panel 1.x api and features.
• Minimum of bugs. Our issues are growing rapidly. We should get to a situation where the list of bugs can be kept to a minimum.
• Community. We need a bigger community of contributors understanding Panel and helping on discourse, helping on github, writing blog posts, making videos etc. for users to be able to get/ find help. We need to communicate it via Conference, Sprints, Blogs etc. We need to get into the documentation of the frameworks that users would use with Panel.
• Make Panel Google Searchable. I often hear that users cannot google Panel help because Panel is such a generic name.
• Make layout and style work. Laying out and styling in Panel is hard. In my experience creating the functionality takes no time. Getting the layout and style right takes a lot of experience and hard work. The most popular, alternative frameworks does much better in this area. Furthermore everything but the most simple styling is really hard to to do after Panel 1.x was released and things moved to shadowRoot.
• Support non-desktop devices. Supporting Panel on mobile is hard. Firstly because there is no documentation or reference examples for this. Secondly because there is no easy way to change the font-size (its not enough to change the --font-size because everything that should depend on --font-size does not. Its hard coded). The consequence is that most times people share a Panel app they open it (on mobile) and it does not look well. That leaves a bad experience.