[BEAM-2437] Input path should be flexible in quickstart

[sb2nov]

R: @aaltay PTAL

The manifest or pom file all will be missing so we should just let the user point to any text file of choice.

[asfgit]

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Jenkins built the site at commit id 8c21e0b 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.

[asfgit]

Refer to this link for build results (access rights to CI server needed):
https://builds.apache.org/job/beam_PreCommit_Website_Stage/539/

Jenkins built the site at commit id 58a9735 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.

[aaltay]

R: @melap

[melap]

LGTM