Add cross-referencing for weaknesses, tests and demos

[cpholguera]

Now that we have all these items with their yaml frontmatter including this metadata:

• weaknesses/
  • with their own id like MASWE-xxxx
• tests-beta/
  • with their own id like MASTG-TEST-xxxx
  • with a field weakness including one MASWE-xxxx ID
• demos/
  • with their own id like MASTG-DEMO-xxxx
  • with a field test including one MASTG-TEST-xxxx ID

Everything is linked. We need a new hook in docs/hooks that will do this cross referencing and automatically add to their metadata (page.meta):

• for a weakness: a list with all tests covering it
• for a test: a list with all demos covering it

If those lists have values it will add a markdown section at the bottom of the markdown file:

• for a weakness: ## Tests
• for a test: ## Demos

For example, for MASWE-0108:

## Tests

- [MASTG-TEST-0206: Sensitive Data in Network Traffic Capture](/MASTG/tests-beta/android/MASVS-PRIVACY/MASTG-TEST-0206)

For MASTG-TEST-0206:

## Demos

- [MASTG-DEMO-0009: Detecting Sensitive Data in Network Traffic](MASTG/demos/android/MASVS-PRIVACY/MASTG-DEMO-0009/MASTG-DEMO-0009)