chore: remove deprecated docs

[flmel]

Part of #2391

• removes deprecated docs
• fixes a small issue on the "not found" page

Algolia will need to be reindexed

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[bucanero]

@flmel I've converted this PR to "draft" just to keep it unmerged until the articles and any legacy content is moved to the new repository https://github.com/deprecated-near/bos-legacy-docs

[flmel]

The removed docs can be found on https://deprecated-near.github.io/legacy-docs/

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