[CI] Semconv: mount README.md as _index pages

[chalin]

• Contributes to:
  • [CI] Render spec pages without preprocessing #6101
  • [editorial] Inline Hugo front matter semantic-conventions#1821
• No longer renames semconv docs/**/README.md to _index.md during the preprocessing step. We instead achieve a similar result using Hugo mounts
• Adds a script to generates the mounts config. I'll eventually hook it up to auto-update hugo.yaml.
• Makes some more changes to the semconv title normalization script, which I've been using to make progress towards [editorial] Inline Hugo front matter semantic-conventions#1821

Command used to generate the mount config:

$ ./scripts/content-modules/print-spec-mounts.pl \
  $(cd content-modules/semantic-conventions && find docs -name "README.md" | sort)
...

[chalin]

/cc @trask

[tiffany76]

RSLGTM.