chore(docs): fixes broken links in Views and Contribute documentation

Fixes #6954
Merge pull request #7222 from juho-jaakkola/views_doc_links

chore(docs): fixes broken links in Views and Contribute documentation
docs(upgrade): clarifies upgrade instructions about updating Elgg codebase

Fixes #7225
chore(docs): replace contents of UPGRADE.txt with a link to learn.elgg.org

Fixes #7226
Merge pull request #7227 from juho-jaakkola/7225-upgrade-instructions

Update upgrade instructions
docs(manifest): document how to use manifest.xml
Merge pull request #7228 from juho-jaakkola/manifest_docs

docs(manifest): document how to use manifest.xml
docs(river): adds documentation on how to use the river
Merge pull request #7229 from juho-jaakkola/river_docs

docs(river): adds documentation on how to use the river
fix(install): confirm that settings.php exists and is readable before including it
Merge pull request #7221 from juho-jaakkola/fix_install_redirect

Confirm that settings.php exists and is readable before including it
chore(i18n): update translations
chore(release): v1.9.2
Merge pull request #7234 from juho-jaakkola/release-1.9.2

Release 1.9.2