doc: fix mkdocs build warnings/relative links

[nodiscc]

Stumbled across this while building the docs using mkdocs 1.5.2, following readthedocs' email Make sure your project is compatible with MkDocs >= 1.x from 2023/08/02

INFO    -  Cleaning site directory
INFO    -  Building documentation to directory: /home/live/GIT/Shaarli/doc/html
INFO    -  Doc file 'index.md' contains an unrecognized relative link 'Usage#tag-cloud', it was left
           as is. Did you mean 'Usage.md#tag-cloud'?
INFO    -  Doc file 'index.md' contains an unrecognized relative link 'Usage#picture-wall', it was
           left as is. Did you mean 'Usage.md#picture-wall'?
INFO    -  Doc file 'index.md' contains an unrecognized relative link 'Usage#import-export', it was
           left as is. Did you mean 'Usage.md#import-export'?
INFO    -  Doc file 'Community-and-related-software.md' contains an unrecognized relative link
           'REST-API', it was left as is. Did you mean 'REST-API.md'?
INFO    -  Doc file 'Community-and-related-software.md' contains an unrecognized relative link
           'Theming', it was left as is.
INFO    -  Doc file 'Installation.md' contains an unrecognized relative link
           'dev/Development#third-party-libraries', it was left as is. Did you mean
           'dev/Development.md#third-party-libraries'?
INFO    -  Doc file 'Installation.md' contains an unrecognized relative link
           'Upgrade-and-migration', it was left as is. Did you mean 'Upgrade-and-migration.md'?
INFO    -  Doc file 'Plugins.md' contains an unrecognized relative link 'Shaarli-configuration', it
           was left as is. Did you mean 'Shaarli-configuration.md'?
INFO    -  Doc file 'REST-API.md' contains an unrecognized relative link 'Server-configuration', it
           was left as is. Did you mean 'Server-configuration.md'?
INFO    -  Doc file 'Reverse-proxy.md' contains an unrecognized relative link
           'Shaarli-configuration', it was left as is. Did you mean 'Shaarli-configuration.md'?
INFO    -  Doc file 'Server-configuration.md' contains an unrecognized relative link
           'Directory-structure', it was left as is.
INFO    -  Doc file 'Shaarli-configuration.md' contains an unrecognized relative link
           'Translations', it was left as is.
INFO    -  Doc file 'dev/Development.md' contains an unrecognized relative link 'Unit-tests', it was
           left as is. Did you mean 'Unit-tests.md'?
INFO    -  Doc file 'dev/Development.md' contains an unrecognized relative link 'GnuPG-signature',
           it was left as is. Did you mean 'GnuPG-signature.md'?
INFO    -  Doc file 'dev/GnuPG-signature.md' contains an unrecognized relative link 'Release
           Shaarli', it was left as is.
INFO    -  Doc file 'dev/Theming.md' contains an unrecognized relative link 'Shaarli-configuration',
           it was left as is.
INFO    -  Doc file 'dev/Translations.md' contains an unrecognized relative link 'Theming', it was
           left as is. Did you mean 'Theming.md'?
INFO    -  Documentation built in 0.40 seconds