Split spec.md into sub files to be able to process terms-and-definitions separately

[henkvancann]

Checked diff on the spec-up generated frontend-side:

https://henkvancann.github.io/tswg-cesr-specification/ versus https://trustoverip.github.io/tswg-cesr-specification/

Result checked and OK

51d50
< <!-- file name: header.md -->
70d68
< <!-- file name: foreword.md -->
73d70
< <!-- file name: introduction.md -->
83d79
< <!-- file name: status-of-this-memo.md -->
88d83
< <!-- file name: copyright-notice.md -->
97d91
< <!-- file name: terms-of-use.md -->
102d95
< <!-- file name: scope.md -->
105d97
< <!-- file name: normative-references.md -->
108d99
< <!-- file name: terms-and-definitions.md -->
150d140
< <!-- file name: composability-and-domain-representations.md -->
277d266
< <!-- file name: text-coding-scheme-design.md -->
497d485
< <!-- file name: table-types.md -->
1120d1107
< <!-- file name: annex-a.md -->
3341d3327
< <!-- file name: bibliography.md -->
3581c3567
<                 <script>window.specConfig = {"title":"Composable Event Streaming Representation","spec_directory":"./spec/","output_path":"./docs","markdown_paths":["header.md","foreword.md","introduction.md","status-of-this-memo.md","copyright-notice.md","terms-of-use.md","scope.md","normative-references.md","terms-and-definitions.md","composability-and-domain-representations.md","text-coding-scheme-design.md","table-types.md","annex-a.md","bibliography.md"],"logo":"https://raw.githubusercontent.com/trustoverip/logo-assets/master/logos/ToIP-Logo-Color-SolidDimensional-Horizontal-LightOnDark.svg","logo_link":"https://github.com/trustoverip/tswg-cesr-specification","katex":true,"source":{"host":"github","account":"trustoverip","repo":"tswg-cesr-specification"},"destination":"./docs/"}</script>
---
>                 <script>window.specConfig = {"title":"Composable Event Streaming Representation","spec_directory":"./spec/","output_path":"./docs","markdown_paths":["spec.md"],"logo":"https://raw.githubusercontent.com/trustoverip/logo-assets/master/logos/ToIP-Logo-Color-SolidDimensional-Horizontal-LightOnDark.svg","logo_link":"https://github.com/trustoverip/tswg-cesr-specification","katex":true,"source":{"host":"github","account":"trustoverip","repo":"tswg-cesr-specification"},"destination":"./docs/"}</script>