This PR does the following:

• updates measurement statistics data to include start_time
• update the charts to use start_time as date for x-axis charts
• add labels for x and y axis and tooltip
• add section descriptions

Note:

• I only found dates (start_time) for data with type sysres. For data with type diskusage, only disk size is available. I couldn't find date info. So I left the charts with commit_numbers as before
• As next step I'll check other linked data repositories (e.g. yocto-buildstats) to see if we can get more detailed data. It can be done when researching for this issue