docs: add hardware sizing section and remove removed MySQL 8 variables from Requirements.md

[somethingwithproof]

Closes #215
Refs #106

Summary

• Add Hardware Sizing section to Requirements.md with a sizing table covering small (< 500 devices), medium (500-2,000), large (2,000-10,000), and very large (> 10,000) deployments, including guidance on:

  • SSD/NVMe requirement for RRD file directory (spinning disk causes polling backlogs)
  • innodb_doublewrite = OFF only appropriate on SSD
  • Spine CPU threading: 1-2 processes per CPU core
  • Remote data collectors for very large deployments
  • Note on memory vs InnoDB engine for Boost tables on large installs (refs Update documentation - Boost table engine for large Cacti installations #106)

• Remove innodb_additional_mem_pool_size from the description list (removed in MySQL 5.7.4; causes startup failure on any current MySQL or MariaDB)

• Remove innodb_file_format = Barracuda and innodb_large_prefix = 1 from the paste-in config block at the bottom of the file (both removed in MySQL 8.0; cause startup failure)

Test plan

• Verify sizing table numbers are consistent with community guidance
• Verify MySQL 8.0 no longer accepts innodb_additional_mem_pool_size, innodb_file_format, innodb_large_prefix
• Verify config paste-in block still works on MariaDB 10.5+

[Copilot]

Pull request overview

Updates Cacti’s Requirements documentation to add baseline hardware sizing guidance and to remove MySQL/InnoDB configuration variables that are no longer valid on modern MySQL versions.

Changes:

• Added a Hardware Sizing section with a deployment-sizing table and operational notes (SSD/NVMe, Spine scaling, remote collectors).
• Removed innodb_additional_mem_pool_size from the MySQL/MariaDB recommendations list.
• Removed innodb_file_format and innodb_large_prefix from the sample my.cnf paste-in block.

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[somethingwithproof]

All three review points are already covered by the current content: the innodb_doublewrite note now spells out the durability trade-off instead of recommending OFF outright, the "Very large" row keeps Disk to pure storage sizing with remote-poller guidance moved to its own note, and the stale PHP 5.4/MySQL 5.6 minimums flagged from #215 are already updated to PHP 8.1 and MySQL 5.7/MariaDB 10.2 elsewhere in Requirements.md. No further changes needed; resolving.