various documentation edits

docs: fix Official External Commands section

download_strategy: Miscalculated redirect when `Location:` is protocol-relative

"occured" -> "occurred"

Replace <var> tags with markup for italicized monospaced text in Markdown.
This requires the source to not combine bracketed and backticked text.

Work around ronn's inability to nest indents within list items by modifying its output to add a line break and indent after lines ending with a colon that aren't a list item's first line. This allows `brew.1.md.erb` to join the commands' help text without extra line breaks, which avoids the issue where kramdown was turning any command specs containing pipe characters into tables in the HTML output.

Mostly edits to formatting, grammar, wording for consistency & readability.

docs: check links with `html-proofer`

Bump codecov from 0.1.10 to 0.1.11 in /Library/Homebrew/test

Bumps [codecov](https://github.com/codecov/codecov-ruby) from 0.1.10 to 0.1.11.
- [Release notes](https://github.com/codecov/codecov-ruby/releases)
- [Changelog](https://github.com/codecov/codecov-ruby/blob/master/CHANGELOG.md)
- [Commits](https://github.com/codecov/codecov-ruby/compare/v0.1.10...v0.1.11

)

Signed-off-by: dependabot[bot] <support@dependabot.com>

[Security] Bump nokogiri from 1.8.4 to 1.8.5 in /docs

Update portable-ruby to verison 2.3.7 for ARM [Linux]

Bumps [nokogiri](https://github.com/sparklemotion/nokogiri) from 1.8.4 to 1.8.5.
- [Release notes](https://github.com/sparklemotion/nokogiri/releases)
- [Changelog](https://github.com/sparklemotion/nokogiri/blob/master/CHANGELOG.md)
- [Commits](https://github.com/sparklemotion/nokogiri/compare/v1.8.4...v1.8.5

)

Signed-off-by: dependabot[bot] <support@dependabot.com>

cleanup: remove vendor/bundle/ruby.

This can be done with `bundle exec rake test`.

Otherwise gem paths will contain references to a now-deleted Ruby.

secure/fix URLs

cleanup: cleanup portable ruby, old linkage cache.

CoreTap.default_remote: Use Linuxbrew/core [Linux]

-march=native is default. Hardware::CPU: Add Skylake [Linux]

setup_remote_tap: use system taps.

- Cleanup portable ruby if you haven't specifically requested it and
  you don't need it. This will be useful when e.g. upgrading OS versions
- Cleanup the old DBM linkage cache. These will not be used again.

This saves recloning the taps we use multiple times when already done by
the user.

cli_parser: man-pages and help text generation using Homebrew::CLI::Parser

azure-pipelines: always upload test results.