Restructure conceptual documentation on Beam site.

[hadarhg]

Restructure and rename "Documentation" tab on Beam site.

[hadarhg]

R: @davorbonaci

[asfbot]

Refer to this link for build results (access rights to CI server needed):
https://builds.apache.org/job/beam_PreCommit_Website_Test/187/
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[asfbot]

Refer to this link for build results (access rights to CI server needed):
https://builds.apache.org/job/beam_PreCommit_Website_Stage/231/

Jenkins built the site at commit id b513534 with Jekyll and staged it here. Happy reviewing.

Note that any previous site has been deleted. This staged site will be automatically deleted after its TTL expires. Push any commit to the pull request branch or re-trigger the build to get it staged again.

[davorbonaci]

Random thoughts:

• Visually, I think we have too many top-level menus. Another solution may be nesting (sub-menus inside a menu). It would probably be less of an issue for a vertical menu, as opposed to a horizontal one.
• Some top-level menus have a top-page, while others don't. We should be consistent.
• Using the documentation is now under Concepts, which makes less sense than before when the title of the menu was Documentation.
• Capability matrix should probably be in its own section, as well as the direct runner. Some intro page would definitely be good here.
• Some menu names are verbs while other are nouns. We should be consistent.

I think all this is ~fine, but it is getting less attractive visually. The only blocking issue is consistency.

[davorbonaci]

@hadarhg, close?