Add a basic HTML page for the default endpoint in ndt7-prometheus-exporter

[fhltang]

This is a small follow up to PR #77 .

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This change is 

[coveralls]

Pull Request Test Coverage Report for Build 193

• 35 of 98 (35.71%) changed or added relevant lines in 2 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-16.0%) to 73.301%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
cmd/ndt7-prometheus-exporter/main.go	0	5	0.0%
cmd/ndt7-prometheus-exporter/web.go	35	93	37.63%

Files with Coverage Reduction	New Missed Lines	%
internal/upload/upload.go	2	96.4%

Totals
Change from base Build 192:	-16.0%
Covered Lines:	766
Relevant Lines:	1045

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💛 - Coveralls