Guides: output valid HTML5

[ci skip]
Fix rails dbconsole for jdbcmysql adapter.
refactor and clean up delete_all method

Now that delete_all with destroy or delete_all dependency behave
the same we no longer need this conditional. This means we can remove
the new delete_all_with_dependency method I added and go straight
to delete_records from delete_all, passing :all and the dependent
directly.
Merge pull request #14985 from eileencodes/refactor-delete-all-method

refactor and clean up delete_all method
Merge pull request #14984 from jkingdon/add-jdbcmysql

Fix rails dbconsole for jdbcmysql adapter
Merge pull request #12667 from nateberkopec/make_guides_html_valid

Fix Guide HTML validation
pg guide, datatypes and examples. [ci skip]
pg guide, advanced features [ci skip]
pg guides, some stylistic changes [ci skip]
Merge branch 'postgres/guide'
Fixed grammar
Merge pull request #14990 from fredwu/has_many_through_association_doc_grammar

Fixed grammar [ci skip]