Add help for new paths

Update information on deprecated backwards naming - issue #648

clarification on running curl as sudo

Co-Authored-By: Mikey O'Toole <mikey@mjco.uk>

Added doc for hook_civicrm_entityRefFilters

civibuild.md - Add documentation about CIVIBUILD_PATH and custom-types

Added to line 22 - "You shouldn't run the curl buildkit command as sudo. It will sudo itself for the bits that need it."

In this page and other `tools/*.md` pages, the sections use basic
capitalization.

Update Hooks Pre and Post documentation indicating Domain is now aval…

civilint.md - Split out comments on multiple lines

These examples are in a code block  - which prevents wrapping. When reading on the published site, you have to scroll horizontally.

Moving them to separate lines makes them more legible.

Updated docs for dev/core#1077

Adding examples to markdown documentation.

Explain difference between ts and E::ts a bit more

Updated docs for dev/core#870

transactions.md - Add more intro. Add API discussion.

Improve hook_civicrm_alterMailParams re: headers and flexmailer. fix #654

Co-Authored-By: Mikey O'Toole <mikey@mjco.uk>

Co-Authored-By: Mikey O'Toole <mikey@mjco.uk>

Co-Authored-By: Mikey O'Toole <mikey@mjco.uk>

theme.md - Add more examples of URL/CSS manipulation

Co-Authored-By: Mikey O'Toole <mikey@mjco.uk>

Co-Authored-By: Mikey O'Toole <mikey@mjco.uk>

Co-Authored-By: Mikey O'Toole <mikey@mjco.uk>