Expound on how matrix exclusion and inclusion work

Use a few more examples, and reflect the following bug fixes
https://github.com/travis-ci/travis-core/pull/339
https://github.com/travis-ci/travis-core/pull/346
Fix typo
emphasize that the private ssh key needs to be used in .travis.yml
Improve wording and formatting in SSH key section
Add first draft of multi-OS doc
Tweak multi-os doc
Comment on `allow_failures`
Mention Homebrew for installing missing software
Merge pull request #70 from nschonni/gh-pages-gem

Convert from jekyll to github-pages gem
perl: fix grammar
Merge pull request #72 from ktdreyer/perl-grammar

perl: fix grammar
Add more detail about statuses in webhook payload
Merge pull request #50 from travis-ci/sf-emphasize-private-key

emphasize that the private ssh key needs to be used in .travis.yml
Merge pull request #13 from travis-ci/ha-build_matrix_doc_improvements

Expound on how matrix exclusion and inclusion work
Merge pull request #73 from connexio-labs/webhook-payload-status-details

Add Detail for Statuses in Webhook Payload
Pluralize 'notification' in section headers
Merge pull request #66 from travis-ci/ha-feature-multi-os

Document multi-OS feature
Add reference to multi-OS doc
Remove French and Brazilian Portuguese files
Escape ampersands