gdal_contour: add page

[JakobMiksch]

Add the CLI tool gdal_contour from the GDAL/OGR suite.

• The page (if new), does not already exist in the repository.
• The page is in the correct platform directory (common/, linux/, etc.)
• The page has 8 or fewer examples.
• The PR title conforms to the recommended templates.
• The page follows the content guidelines.
• The page description includes a link to documentation or a homepage (if applicable).

Version of the command being documented (if known):
3.2.2

[reinhart1010]

It might be better to place some mnemonic tags here, as gdal_contour unfortunately doesn't put longer parameter names (e.g. --attribute instead of -a) which are more preferred in writing tldr-pages.

[JakobMiksch]

@reinhart1010 thanks for your review, I applied your suggestions.

[marchersimon]

Looks good, thanks

[reinhart1010]

Thanks!