[foreign-jextract] RFR: Consolidate all the various documents in the repository.

[Maurizio Cimadamore]

This patch adds all the main documents (such as State of foreign memory access, State of foreign function access) to
the repo as "live" documents. Together with the existing jextract document (which is also revamped a bit), each
document clearly illustrates the charateristics of each main branch in the repo.

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Commit messages:
 - Add html docs for memaccess/ffi
 - Update main README with links to foreign linker JEP
 - Fix memaccess doc
 - Experiment with <small>
 - Fix <sup>
 - Further tweak footnotes
 - Refine footnotes
 - Normalize footnotes
 - Tweak doc
 - Improve jextract doc
 - ... and 3 more: https://git.openjdk.java.net/panama-foreign/compare/96b61170...4a37c655

Changes: https://git.openjdk.java.net/panama-foreign/pull/355/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=355&range=00
  Stats: 2327 lines in 7 files changed: 1528 ins; 31 del; 768 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/355.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/355/head:pull/355

PR: https://git.openjdk.java.net/panama-foreign/pull/355