Rewrite the pages about metadata in the docs

[soininen]

This replaces "Metadata description" and "Result metadata description" pages in the documentation by a single "Metadata" page. The removed pages contained a datapackage-like JSON definition that is probably too ambitious to our real needs. The new page contains merely a short introduction to metadata and some suggestions on what kind of stuff to store there.

Resolves #329

Checklist before merging

• [x] Documentation (also in Toolbox repo) is up-to-date

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (a97c794) 77.12% compared to head (2563b3b) 77.12%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## 0.8-dev #340 +/- ## ======================================== Coverage 77.12% 77.12% ======================================== Files 75 75 Lines 9241 9241 Branches 1950 1950 ======================================== Hits 7127 7127 Misses 1781 1781 Partials 333 333 ```

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