No sitemap no robots

[Bromind]

Addresses feature request #2248

this simply adds two fields in config.toml, one field no_sitemap, which disable the generation of sitemap.xml if set, and one field no_robots, which disable the generation of no_robots.txt if set.

IMPORTANT: Please do not create a Pull Request adding a new feature without discussing it first.

The place to discuss new features is the forum: https://zola.discourse.group/
If you want to add a new feature, please open a thread there first in the feature requests section.

This feature is discussed in #2248 "We could make it a config.toml option." (Keats, July 2023) -> This is exactly what this PR implements

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Code changes

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• Have you created/updated the relevant documentation page(s)?

[Bromind]

messed up with my PR, I'll create a new clean one