Updates to Raw data format pages

[emiliom]

• Generalize the name and title of data-format-5to6 page
• Add section describing 0.7.1 to 0.8.0 major changes
• Fix some broken links, including the SONAR-netCDF4 v1 report url
• Add section that stub that will includes a brief reference to the echopype-checker package and repo. Note that I've already renamed the repository (and package) to echopype-checker and made it public. But I haven't overhauled its README file yet.

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[leewujung]

@emiliom : I made the following changes via direct push:

• make mentions of "version 1" and "v1" consistent: all changed to "version 1"
• remove "and is required for these two variables": I assume you are referring to the requirement in the convention, but in the context of this sentence it is a bit hard to decipher; and I think the key info in this sentence is complete without this subsentence.
• change everything to present tense to be consistent with release notes (since there are some tricky cases where past tense does not make sense when describing data files in general)
• make bullet point format more consistent: "In XYZ group: ..."

[leewujung]

Thanks @emiliom ! I'll now merge this.

[emiliom]

Thanks! Your edits look good.