Revamp Internal Horizon Documentation

[aditya1702]

PR Checklist

PR Structure

• This PR has reasonably narrow scope (if not, break it down into smaller PRs).
• This PR avoids mixing refactoring changes with feature changes (split into two PRs
  otherwise).
• This PR's title starts with name of package that is most changed in the PR, ex.
  services/friendbot, or all or doc if the changes are broad or impact many
  packages.

Thoroughness

• This PR adds tests for the most critical parts of the new functionality or fixes.
• I've updated any docs (developer docs, .md
  files, etc... affected by this change). Take a look in the docs folder for a given service,
  like this one.

Release planning

• I've updated the relevant CHANGELOG (here for Horizon) if
  needed with deprecations, added features, breaking changes, and DB schema changes.
• I've decided if this PR requires a new major/minor version according to
  semver, or if it's mainly a patch change. The PR is targeted at the next
  release branch if it's not a patch change.

What

Closes #4936 and #4939

This PR rearranges and adds internal documentation:

• Creating an updated development guide that is easier to understand.
• Adding a new readme on Horizon SDK and API.
• Adding references to all these files on the main README page for easier access.

Why

There was a lot of redundant readme files and documentation that made it harder for someone new in the team to understand the codebase. These changes remove the duplicate documentation, adds some new READMEs and brings all important resources under a single roof.

Known limitations

None.

[sreuland]

the new consolidated content looks good, thanks for addressing all the feedback, I left a few additional suggestions on minor tweaks.