Move all mentorship pages into Mentorship section, and other docsy prep

[chalin]

• Contributes to Migration to docsy theme #746 by doing some prep work.
• Relocates all mentoring pages, placing them under the /mentoring/ section (that already exists)
• Adds aliases so that the old paths will redirect to the new page locations
• Adds a few other Docsy related config and shortcodes

The only change introduced by this PR is the relocation of the mentorship pages:

$ (cd public && git diff -bw --ignore-blank-lines -I "build-timestamp|mentorship.for-m" -- ':(exclude)*.xml' ':(exclude)*.(map|js)') | grep ^diff | head
diff --git a/docs/index.html b/docs/index.html
diff --git a/mentorship-for-mentees/index.html b/mentorship-for-mentees/index.html
diff --git a/mentorship-for-mentors/index.html b/mentorship-for-mentors/index.html
diff --git a/mentorship/index.html b/mentorship/index.html

The change /docs/index.html can be ignored since it's impossible to reach that page in the current site config (that page redirects to the current version's doc landing page.)

Previews:

• https://deploy-preview-998--jaegertracing.netlify.app/mentorship/
• https://deploy-preview-998--jaegertracing.netlify.app/mentorship/for-mentees/
• https://deploy-preview-998--jaegertracing.netlify.app/mentorship/for-mentors/

Redirect tests:

• https://deploy-preview-998--jaegertracing.netlify.app/mentorship-for-mentees/
• https://deploy-preview-998--jaegertracing.netlify.app/mentorship-for-mentors/